Noise during acceleration

2002 Mercury Sable with 3.0 Vulcan, NOT dual overhead cam. Basic motor. I have a flapping or thumping sound from the motor under the hood. Only happens when you accelerate and in gear. Only happens when in drive, goes away if in neutral. In neutral can rev the motor up and down without any flaping sound, no flap or thump. Continuous flapping sound at all speeds as long as you have your foot on the gas and are accelerating. Take your foot off and coast the sound goes away. Give it some gas and the sound is back. Not dependent on motor RPM, but does increase in flap speed as your car speed increases. Never heard of this before, and this is the closest thread I could find on the Web. Please help!

Welcome to the forum!

Do you notice any difference in the power of the engine? When is the last time the engine had a tune up? Are you familiar with misfires and can state that this is not a misfire? Please don't be offended by this question as I have no idea what your level of experience is with working on or diagnosing cars.

-Rod

Could be worn motor mounts causing interference with moving parts of your engine or transmlission..

If it's wheel speed and load dependent, I would guess driveline, specifically inner CV joints- check for torn boots, and excessive play.