I have two identical PCB's

PCB A functions normally
PCB B when powered up has a ton of noise on the 5volt rail and the 12 volt rail, I assume this is a ground loop problem?

Ground is the top right pad, a 5 volt regulator is just to the left of it.

Just to the left of that, a .01 uf cap, 33pf cap, 200k ohm resistor, then a dip 8 IC.

The line below the +12v line is an audio line fed from a portable MP3 player / iphone. The shield from the audio cable is soldered to the ground plane on the PCB.

Here are two pictures of the 5v line.
PCB A is the first one with minimal noise.. This doesen't cause any problems..

This is PCB B everything connected the same, and this one has a ton of noise on the 5v line. Any ideas?

 

Looks like something is oscillating. I would look for missing or faulty filter caps.

 

Found It!

Ended up being a faulty transistor on one of the output lines that was causing the noise. I triggered them individually with a test micro and noticed the noise only appeared when one pin was pulled high. The only thing else on that trace was a resistor and transistor, removed/replaced the transistor and the problem went away!