When my truck shifts into 4th gear it often makes a noise similar to the sound of a semi when it is engine braking. After it picks up a little speed it goes away. It also seems to very slightly bog down immediately after the shift and then picks up speed. The truck is a 3.9v6 with a 42RE I believe. Any ideas?

 

Check your fluid level in your transmission you may be low. The noise you hear may be the overdrive band slipping. When was the last time you had the trans serviced and the bands adjusted?

 

I just bought the truck a few months ago so I haven't really done much to it. When I first got it, shifting was a little erratic as well and the idle would sometimes go up and down for no reason. I changed the speed sensor and that seems to have fixed some of the problems. The first time the noise happened it kind of freaked me out. I have never experienced anything like it before. Is it easy to adjust the bands? I did notice that the transmission mount is a little messed up with the rubber kind of hanging out one side. Do you think that could cause the noise?

 

Adjusting the bands is part of regular servicing of the transmissions on these trucks. It isn't hard to do but you have to take off the transmission oil pan to do and it will get a bit messy dropping the transmission oil pan. Any repair book will tell you how to do this, or a search of other threads may have the procedure laid out. First check the trans level to be sure it is correct, low fluid will cause lots of issues for the transmission shifting and is the cheapest thing to do. Rubber hanging out of your mount is a pretty sure sign it is done. Get a replacement mount to ensure your drive train is properly supported.