2001 GMC Duramax (72K); immaculate shape and running perfectly. On the road, if I put any air on....just vent or actual a/c...and DO NOT place it on "recirculate"...aka "MAX"....all sounds fine. I push the "recirculate/MAX" button, and a fan sound starts cycling from the engine area. No noticeable change in power or anything....just comes on for 3-4 seconds, then off for 5-7 seconds or so.

???

--Bryant

 

Hmm.. Have you had your A/C system checked? Im not sure why it would make any different noise just by recirculating.. But I would have it checked and make sure its full and has no leaks.