Hello! New member here, and fairly new to Fords generally and F150s specifically (though I was consider myself an extremely capable amateur mechanic, having done mechanical and body maintenance and repairs on vehicles for decades).

I've got an issue with a somewhat-new-to-me truck. It's a 1991 F150, 2WD, 4.9L, AT, bare-bones truck. I've not had all that long and it has an unknown service history. It had been sitting a long time and showed it. When I got it, I changed all the fluids and filters, replaced the brake master, front calipers, rear wheel cylinders, new plugs/cap/rotor, radiator, water pump, coolant hoses, etc. After that, it was and still is driving great. However.....

On acceleration, in say 1st & 2nd gear, I get a clicking noise that I can only describe as sounding like that electrical popping sound you get when you hold a spark plug wire near metal and turn the engine over. But it's not completely rhythmic, it's sort of rapid-fire random. There might be two or three or ten in a burst then silence for a moment then a single pop or two then silence and then a burst, etc. Within bursts it is rhythmic. If I lift the throttle it stops. If I'm cruising steady on level ground so that the engine is lightly loaded, it doesn't do it. It seems more tied to road speed than engine speed. And it sounds as if it's coming more from the rear of the truck than the front. If I had to guess at a location I'd say around the diff. While it's happening, I feel nothing in terms of driveability issues. There's no physical sensation such as a surge or buck or miss or vibration that corresponds with it. If you didn't hear it you wouldn't know anything is going on.

I've been unable to reproduce it by loading the engine while not moving. And it started doing it recently -- was not making this noise after I had first recommissioned it.

I've never encountered anything like this and I'm scratching my head trying to imagine what could possibly be causing it. I can't think of anything that would cause such a noise (and not be felt!) Would there happen to be any commonly-known issues that cause this sort of noise, or something even vaguely similar?

Thanks

 

When you replaced cap and rotor, did you mess with the distributor? It almost sounds like spark knock. Pull a couple plugs and check the porcelain. If it's knocking you can do major damage to the pistons.

 

Thanks! I doubt seriously it's spark knock. I know what that sounds like and this is different. And as I said, it sounds as if it's coming from the rear of the truck. And it's only started recently -- like within the last 100 miles or so. Didn't do it for the first 4000 miles or so since I did the major service work. But you're right that spark knock would occur under the conditions I'm hearing this noise. Hmm.

 

Might be worth checking the braces from the bottom the bed, to your rear lower quarter panels; your exhaust pipe and your leaf spring shackles.
When I put the muffler on my '88, I had to rotate it to keep it from hitting the bottom of the bed, because the inlet and outlet of the muffler are not concentric.