I've had my 2006 XLT F150 (5.4L) for about a month now and have recently noticed what I can only describe as a 'smacking' sound under my truck when I accelerate quickly. It sounds as if I had something stuck to my drive shaft that is slapping the underside of my truck....except I don't. I'm perplexed at what this could be. Any thoughts?

 

Could be your timing chain slapping around. It could be loose.

Double check to make sure some of your friends didn't put a plastic tie strap on your driveshaft. I've done this to a guy many years ago and it make quite a racket.

 

I don't think that its the timing chain because it seems to be coming from under the cab (between drivers/passenger seat maybe). There doesn't seem to be anything hungup on my drive shaft that would cause the noise. I hear it most often if I try to quickly accelerate from a normal speed (50mph or so) to pass another vehicle. I do not hear it otherwise. Starts with a 'tink tink' then sounds like something hitting the underside.

 

Interesting....I can't think of anything right now.
Will it do it sitting stationary in park and revving the motor?
If so, have somebody rev it for you and get under the hood and see if you can hear it.

Is it a fast slapping that is steady pace? Or does it start and it's just hit or miss during acceleration?

 

I just tried it in park, and I might have heard a slight tick but I'm not sure. (coulda been me just waiting to hear it) It seems to only happen if I aggressively accelerate, as one would to pass someone) there's a slight hesitation then a few "tick tick" sounds. The "slap" seems to be hit or miss because I couldn't replicate it. I can accelerate at a normal pace, or even a fast one without this happening. Only when I really get on it do I hear it.

 

Oh and I tried it in 4x4 to rule out vacuum and it still happens, and even seemed a bit worse. Hesitation and ticking.