single "clunk" when starting the engine
 

Hello,

2017 F-150 with 3.5EB, 4x4, electronic locking diff, about 1600km only. Not too long ago I have noticed an unusual noise - a single "clunk" within a second from starting the engine. It seems to be coming from the back, although I am not sure. Does not happen every time, seem to be something like 1 out of 6-8 starts or so. I am not sure if there is a strong correlation, but it seems that it happens if I park on a slope or if I park by backing up. I do not hear that "clunk" when starting off the complete stop on the street, even if auto start/stop was on - I hear it only when restarting the engine after parking. Does not seem to matter if the engine is cold or warm, heard it in both cases.

I did some research and the only thing I have found was mentioning of a TSB 04266 (very old) and the possible need to lubricate the slip yoke.

Going to see the dealer in a couple of days but I'd be curious if anyone had a similar situation with recent F-150s.

You are hearing this without putting truck in gear? Right after the engine starts?

Yes. Transmission is in 2H mode, in PARK, manual brake on. Right after the engine starts.

One of my thoughts was that it is something mechanical, like exhaust touching another part or something else that touches the skid plate...but usually, these sort of sounds reoccur while driving or idling, I think. This sound - I only hear it once and not every single time I start the truck.

Backing in a spot on a sloop. If so before shutting off engine and putting in park try this. Apply parking break place in park shut off engine. Next day start engine put in gear then release parking break. Sounds more like the parking break popping free after it is parked to my internet ears

I was thinking about the parking brake being somehow involved, but it did happen a couple of times even without applying it - after parallel parking on a flat street.

Silly thought....One of my old cars (BMW X3) had a mechanism in the exhaust system, like a spring-powered gate of some sort. When starting it cold, it used one exhaust pipe, then a few minutes later it opened the second one with a loud metal "bang". It was by design. I am wondering if F-150 may have something similar. Although in my F-150 it happens with both cold and warm engine.

If it's not bucking, just the clunk, it has to do with the auto stop start system. It will make one clunk when the engine is warm and you start it

Possibly the AC compressor kicking in?

Thanks for the hint. I am going to test the theory. I think it is possible - since I cannot really correlate this sound and any other conditions (driving/parking in a particular way, using/no using the hand brake etc). But, indeed, when A/C is on or when the system is on AUTO we do not know when the pump is running or not...I am going to continue testing it.

BTW, went to the dealer but they "could not reproduce it". I believe the guy did not even look underneath, just tried to start/stop the vehicle couple of times. To be fair, I could not reproduce it either so it is hard to troubleshoot something in this situation.

exhaust tight? manually try to shake it for movement.

On my 2015 5.0 4x4 I get a clunk at different outside air and engine temps just after engine start . Not every time and can do it after remote start.
" Was the radiator shutter. " I found it because I started truck while standing in front near grill. While in drivers seat the sound of shutters closing sounded like coming from somewhere else. Could this be the sound you hear ?