“thump” beneath floor at idle
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Joined: Apr 2020
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“thump” beneath floor at idle
Hi everyone! I’m new to this forum and also the new owner of a 1977 F150 Custom. It’s a 4x4 short bed with a 240 inline 6 and NP 435 transmission. About 220k on the chassis and 80k on the engine according to the previous owner. I picked up this truck five days ago for a pretty good price - maybe too good, as it turns out, because a few things have started to act up in the past 24 hours.
Here’s a link to a video of the sound I’m dealing with - in case anyone wants to skip my long story below.
—
First, three days ago I noticed a small oil leak, coming from mainly between the engine and bell housing. It didn’t look terrible but I made a note of it. The morning after noticing this I noticed the engine smoking a bit more than usual at cold start. Hmm... notice a pattern? Yesterday, started hearing some ticking while driving that sounded like the engine needed oil. I pulled the dipstick when I got home and, sure enough, the truck was 2 quarts short on oil! The oil that’s in the engine currently looked a bit thin so I topped it off with Shell Rotella 15-40 after reading some advice on this forum, then let the car sit during the day.
That evening, I started the car to drive to my girlfriend’s house and immediately started hearing a “thump”-ing from what sounded like the rear of the engine. It was rhythmic and intermittent and I could feel it clearly through the floor - it sounded just like someone rapping their knuckles against the bottom of the truck. I shut the car off, looked under the hood, didn’t see anything. Oil looked good, no leaking beneath the car. Nothing in the coolant, which looked clear also (PO probably changed it before sale, though). When I cranked the car over again it clunked a few more times and then mostly quieter down while I drove.
This morning I started the truck and the clinking was back again, but much louder. I felt a bit helpless and, not knowing these engines well yet, decided to get it towed back to my house. When the tow driver showed up, i turned the engine over for him to see and, mysteriously, no clunking. He said the engine sounded perfect and jokingly offered to buy it off me if I was that scared, lol. I drove the truck home and this time felt somewhat convinced the sound was coming from the suspension, as I only heard creaking & thumping when starting to move from a stop. Took the highway & drove up the steep hill to my apartment without issue.
Finally, this evening I got in the truck again and decided to record the sound. From cold the noise was quiet and intermittent, like this morning. After about ten minutes of driving the sound got quite a bit louder and much more rhythmic. I took a video to record the sound from inside the cab, which is linked above. From under the hood the noise isn’t audible and from beneath the car it barely registers as a slight tapping, but from inside it’s clearly heard and felt.
I’ve never heard an engine noise like this before and am having trouble finding anything similar on the internet. My thought was that it must be something in the engine but now I wonder if it might be a failing motor or suspension mount, or maybe an exhaust problem? I’m really perplexed and am curious whether anyone is able to point me in the right direction. Thanks so much for the help, looking forward to being part of this community!
And here’s a picture of the truck
Here’s a link to a video of the sound I’m dealing with - in case anyone wants to skip my long story below.
—
First, three days ago I noticed a small oil leak, coming from mainly between the engine and bell housing. It didn’t look terrible but I made a note of it. The morning after noticing this I noticed the engine smoking a bit more than usual at cold start. Hmm... notice a pattern? Yesterday, started hearing some ticking while driving that sounded like the engine needed oil. I pulled the dipstick when I got home and, sure enough, the truck was 2 quarts short on oil! The oil that’s in the engine currently looked a bit thin so I topped it off with Shell Rotella 15-40 after reading some advice on this forum, then let the car sit during the day.
That evening, I started the car to drive to my girlfriend’s house and immediately started hearing a “thump”-ing from what sounded like the rear of the engine. It was rhythmic and intermittent and I could feel it clearly through the floor - it sounded just like someone rapping their knuckles against the bottom of the truck. I shut the car off, looked under the hood, didn’t see anything. Oil looked good, no leaking beneath the car. Nothing in the coolant, which looked clear also (PO probably changed it before sale, though). When I cranked the car over again it clunked a few more times and then mostly quieter down while I drove.
This morning I started the truck and the clinking was back again, but much louder. I felt a bit helpless and, not knowing these engines well yet, decided to get it towed back to my house. When the tow driver showed up, i turned the engine over for him to see and, mysteriously, no clunking. He said the engine sounded perfect and jokingly offered to buy it off me if I was that scared, lol. I drove the truck home and this time felt somewhat convinced the sound was coming from the suspension, as I only heard creaking & thumping when starting to move from a stop. Took the highway & drove up the steep hill to my apartment without issue.
Finally, this evening I got in the truck again and decided to record the sound. From cold the noise was quiet and intermittent, like this morning. After about ten minutes of driving the sound got quite a bit louder and much more rhythmic. I took a video to record the sound from inside the cab, which is linked above. From under the hood the noise isn’t audible and from beneath the car it barely registers as a slight tapping, but from inside it’s clearly heard and felt.
I’ve never heard an engine noise like this before and am having trouble finding anything similar on the internet. My thought was that it must be something in the engine but now I wonder if it might be a failing motor or suspension mount, or maybe an exhaust problem? I’m really perplexed and am curious whether anyone is able to point me in the right direction. Thanks so much for the help, looking forward to being part of this community!
And here’s a picture of the truck
Senior Member
Joined: Oct 2016
Posts: 3,700
Likes: 766
From: Parts Unknown
Use a stethoscope and start probing the lower end of the engine and exhaust, while taking all safety precautions. I would also look at engine and transmission mounts. Have a helper apply the e- brake and hold on the brakes while in gear and have him goose the throttle while you observe if the engine wants to jump out of its craddle. Also check the exhaust, CAT conv. heat shields and if there is a rattling inside of the CAT.Go back to the tailpipe and see if the noise or sputtering is coming out the exhaust which I believe it is from a misfire.. Noises are difficult at best to diagnose on a forum, you will just have to crawl under and start investigating, from your video, this should be easy to determine the source of the babbaloos. From your video, I believe it has to do with exhaust (misfire?)or something is banging underneath, not a rod or bearing from the engine..
Last edited by raski; Apr 12, 2020 at 07:11 AM.
Thread Starter
Junior Member
Joined: Apr 2020
Posts: 5
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Sorry for the radio silence! Thanks so much again for the help - my laptop broke a couple days ago and I’ve had to recover all my passwords onto my phone manually lol 
I took your advice and quickly figured out the source of the problem: the muffler is held on by two weak, old looking clamps and was rotating around in its cradle and hitting the truck bed at idle. Thanks for reminding me to go look for myself before spending hours reading forums
But then I went through and did the other tests you mentioned and found a couple more issues. The motor mounts are definitely weak; a strong rev pulls the whole engine down about three inches on the pass side. And there’s some loose rattling from the transmission, both at idle and starting around 2000rpm in 3rd. In both cases it quiets down by holding the shifter ****. I’m guessing the idle rattling is a loose clutch and the in-hear issue might be with the synchro? This is my first time working on a transmission so i’m going to go in and see for myself what’s going on when i have a couple days off next week. Again, really grateful for your help!
I took your advice and quickly figured out the source of the problem: the muffler is held on by two weak, old looking clamps and was rotating around in its cradle and hitting the truck bed at idle. Thanks for reminding me to go look for myself before spending hours reading forums
But then I went through and did the other tests you mentioned and found a couple more issues. The motor mounts are definitely weak; a strong rev pulls the whole engine down about three inches on the pass side. And there’s some loose rattling from the transmission, both at idle and starting around 2000rpm in 3rd. In both cases it quiets down by holding the shifter ****. I’m guessing the idle rattling is a loose clutch and the in-hear issue might be with the synchro? This is my first time working on a transmission so i’m going to go in and see for myself what’s going on when i have a couple days off next week. Again, really grateful for your help!
