Identifying a cracked exhaust manifold
 

Does it make some kind of sound, or can you smell fumes?

There's many signs. A ticking sound, when you sea foam you'll actually see the smoke coming out the manifold, rev is a bit louder, and if I'm not mistaken rpms flutter a bit.

Mine has a ticking sound, but only after start up. It gradually goes away completely.

Seafoam then you'll know for sure sounds like you got a leak. Now there are ways to fix this problem, you can upgrade to a full exhaust, long tubes . Or get it welded and or new shorty headers

I doubt your manifolds cracked. Sounds like you just have an exhaust leak, probably where the manifold connects to the downpipe. Pretty common of these motors.