Chevrolet: noise at 30 mph, chevy silverodo, hd 1500

Question
Hi,Roger thanks first off for this site. I have a 2001 Chevy Silverodo HD 1500 2WD. When driving thru town only at 30 mph you can here a noise coming from what seems to be the mid section of the truck. The noise of course is hard to explain but seems almost like a gurgle more than, say clunks & clangs. I am 100 % sure it is not a engine related noise. When in nuetral you can rev. the engine and everthing sounds great. lastly i would like to mention the car never makes this noise at any other speed, except occasionally in drive from idle to 5 mph. Thanks Rich.

Answer
Hi  Rich ;
        I wonder if it only makes the sound when you go 30, do not go 30 OK.lol no only kidding- I wonder if it could be from the  cat. converter . as you say no engine sounds . have you looked under there for any wear marks or loose part etc. I have a 99 silverrodo 4x4 and that makes some sounds. I would go under and give a good look ./ see if the   return tank looks ok.