So just bought a 2006 ford f150 with 22000 miles on it, had the alternator replaced recently, not sure if this is related but when accelerating steadily as opposed to punching it, right at about 2500 rpm's i feel this slight grinding sensation on the pedal and hear it, at about 2800 rpm it dissapears and the car shifts fine, however when i let go of the gas pedal as the rpm comes down from 3000 to 2500 it makes that sound again at exactly 2500 rpm. I took it into aamco transmission and they ran diagnostics and couldn't find anything, so my car is basically sitting at carmax where i purchased it and they are going to check it out, any suggestions would be greatly appreciated. I'm kinda desperate right now, cause carmax keeps telling me its fine, but i recently took it into the ford dealership and test drove it with one of their mechanics and he said its either coming from the transmission or the rear engine, so i know def. there is something wrong.

 

I used to get the same sound you're talking about when trying to start my truck. The transmission flex plate was the culprit for me. good luck!

 

I used to have that sound on the Rental F150 I had. And like hockey, only at start up. After about 6 start ups, it went away.