My 2001 F-150 4R70W transmission with 107,000 miles on it is making a growling/grinding noise. However, it shifts fine. The noise is only audible in the cab and increases with speed, truck can be in neutral coasting and it makes the same noise. So I thought at first it was the wheel bearings or brakes hanging up, I checked all that and found nothing. I then put the rear end up on jack stands and placed the transmission in drive (with friend in drivers seat). I listened to the differential through a broom stick (crude, but somewhat effective). I only heard fluid slooshing around. I then placed it to the transmission extension housing and this seemed very loud to me. How much noise will the extension housing make? However, you can't hear the noise with the naked ear when the truck was on jack stands and in gear. It is very hard to pinpoint where the noise is coming from, but if I had to guess it is coming from under my feet when I am driving. How can I be sure it is the extension housing vs. the transmission. Any suggestions? This is driving me nuts!

Thanks - Jeff