sounds like an exhaust leak
 

my 2000 gmc sierra sounds like an exhaust but when im underneith the truck i cant hear anything but when im driving around i can hear it and it sounds pretty loud
could it be leaking from my manifolds?

The best thing to do is have someone start it cold cold and get under to see if you feel it or hear it.

ive done that with the remote start and i can hear it but i just cant locate the leak
and my service engine soon light came on and the code said low efficency catilist system bank 2
could this be from the exhaust leak or do i need new cats?

Ohhhh well it could be a bad down stream O2 sensor or one or both of the cats is bad and clogging up. If its clogging up then it will sound like its leaking exhaust somewhere. While its running does the exhaust stink real bad?? Go to the tailpipe and have someone rev on it and it may sound like its clogged up.

what kind of noise should i be looking for when i do this to tell if its clogged?