I have an '87 K5 w/ 4" lift and 33's. A few months ago I went from 3.08 gears to 4.10s. When I first drove it home the truck made a growling noise when I let off the gas at around 65-70 mph. Now, it will do at speeds as low as 35 mph (sometimes). It has also started making a growling noise at speeds above 60 without letting off the gas. It won't always do it. Sometimes it will growl for a little while and then quite down and sound fine, and then a little later start growling again. Any ideas?