I dunno about common, only people I've heard of having it are ones that don't do regular maintenance and people with CRAPPY MECHANICS!!!!! haha..ok..I'm ok...
The quick and nasty way of telling if its rod knock is to take it up to about 2500rpm and do a VERY quick 1/2 throttle jump. If you hear a knock when you do it thats it. It'll sometimes also show its nasty face when driving with light throttle above 2500rpm.
If its something in the valvetrain it'll usually go away above about 1500rpm. If its only there, or loudest, when cold it may just be pistons slap which is the only "normal" knocking noise I can think of.
Stuff got done.