Hey guys, I've got a strange issue with my 2005 F-150 4.2L 112K miles, 2WD, auto. Hoping someone can point me in the right direction. Symptoms:

1) Engine runs fine, no power problems, no CEL
2) When cold, no abnormal sounds
3) Once warmed up, in park, slightly pressing the gas produces a faint tapping sound, similar to a collapsed lifter
4) Once warmed up, putting the truck in drive while holding down the brake and slightly pressing the gas produces a louder more pronounced tap
5) Removing the accessory drive belt eliminates the tap completely
6) Tap changes with RPM
7) I removed the fan clutch assembly and I still get the sound described in both #3 and #4 above
8) Turning the A/C compressor on/off makes no difference
9) Seems like the tapping is not there if the gas pedal is "stabbed" or while driving at speed

I thought for sure this was a collapsed lifter or a worn rocker so I went and bought all the parts but pulled the accessory belt prior to check, and sure enough the tap can not be reproduced without the belt. I am familiar with what a lifter tap sound like, and this sounds exactly like it. So far I have done the following:

1) Changed all rockers and pushrods (prior to realizing the noise goes away with the belt removed)
2) New belt tensioner
3) Two new idler pulleys
4) New water pump
5) New belt

So essentially, the only remaining original accessories being driven by the belt are the alternator, A/C compressor, and power steering. I can remove the belt and hand spin/feel all the pulleys and all seem ok. The power steering pump moves slightly in and out, I read somewhere this is normal. I don't see any crazy wobble of the harmonic balancer and the motor feels balanced and smooth when running. As I mentioned, running the engine without the accessory belt (so only the crank / harmonic balancer is spinning) and poof, the sound is gone. I am not a fan of throwing parts at a problem, but this one has me stumped. Hoping someone may have some experience with this. I don't want to spend money on an alternator, A/C compressor, etc unless I have to. I have also considered sanctioning and exorcism, but that is also expensive and doesn't guarantee the cure. Appreciate any advice!

 

Can you post a video with sound?

 

Can you get a helper to work the gas pedal while you poke around CAREFULLY under the hood with a stethoscope?

Did you replace the belt?

 

Thanks for the quick replies. Yes, I did put a new belt on, I forgot to mention in the original post. I've added three videos, had to edit video quality and length to fit within the 5 MB limit, but I kept the full quality audio track for your listening pleasure. 1. Cold with belt (no tapping), 2 Warm with belt (tapping), 3 Warm without belt (no tapping). The tapping sounds the same from both sides of the engine, the video was cut short due to size limitations so you will not hear when I put the camera behind the alternator. The warm without belt video was 5 seconds after warm with belt video. I really appreciate everyone's time, hopefully we can all learn something from this in the end.

NOTE: don't mind the missing bolt from the A/C bracket. I broke it during removal when doing the water pump. The noise was there before and after. I will replace once its determined if the A/C and power steering need to come off. Cheers!

I'll check with the stethoscope and update any findings.

 

Stethoscope, or as I saw one old mechanic use a long screwdriver as a stethoscope, or piece of hose to narrow it down to the component. I would suspect a loose bolt possibly in the mounting of one of the accessories. I had a noise similar to this waay back when on my 65 Chevelle that turned out to be a cracked alternator support.

 

Videos posted. Thanks again!

 

A wild guess is the AC compressor because it seems to be loud on the driver's side.