Why does my truck run funny once in a while

I bought a 2000 Chevy suburban 2500 last year. Got full tune up on it battery.
It's
my secondary car so I don't drive it as much. But when I do it makes a weird
rubbing sound like a helicopter is taking off inside the truck. I noticed it when I
got stuck in traffic a few times.  When I brought it to the dealer they said fan
clutch. Got that fixed and a week later same sound. Brought it back, they had
it
for a week and said sorry we can't find anything. I drove it for a few days to
see
if I could make it happen again and nothing happened. Then just the other
day it
happened for a few min.  I have no clue. Any ideas?