weird sound when gassing up

[chinoxl37]

jus had some body work done on car.. will post pics in next thread... but ever since i got the car back everytime i gas up i heard a weird noise constantly coming from which sounds like the muffler... its like a quiet vrooom noise... u sometimes hear it on cars that has their exhaust done... its not overly loud but its constant.... basically it sounds like a quieted muffler... nothing is blocking my exhaust cuz when i go check i see smoke coming out and feel the heat.. could the pipe feedinging into my muffler be cracked and causing that noise or could it be possible that i have a blown head gasket? if i do how can i check besides lookin for white smoke after warmup? could it be air filter? thanks in advance