Single LOUD pop from engine bay! Any ideas?
 

So i was driving on the highway today... and something happened.
A little history on my car. I just bought it, got it tuned up, replaced ignition coil, breakes, etc... It is leaking oil into the engine bay, causing smoke (i BELIEVE, my mechanic told me that.)

So i was driving, on the highway, about 3k rpms... when all of a sudden i heard the sound of metal on metal? but really quiet... i couldn't make it out really. coulda just been the wind, cuz it wasn't pronounced enough. Turning is not harder, or any different... Drove the same as it had in the first place, only thing is, i bottomed out the day before... on graval.

When it popped, i checked under the hood, nothing... under the car... there was a hold in my exhaust manifold, leaking and smoking. I know that could have caused the pop... but my mechnaic had siad something was weka, and would POP really loud and i'd hear it. jsut dont know what it is. Any ideas? Don't be biased, disregard his diagnosis if needed!

Temp was ok, and drove fine after.