I have an '01 4.3 with 45,000 miles on it. When I am at a stop and take off, I hear a fluttering noise coming from the the engine compartment (I think?). After about 2nd gear I can no longer hear it, whether it is because it stops or the engine gets too loud I don't know. I took it in and the shop said it was the rearend, so they rebuilt it (luckily I got the ext warranty). But when I got the truck back today it is still doing it. Any ideas what this is or has anyone else had this problem? It started about 500 miles ago.