Diy helph

Nice one fuzztone, 
assuming that your connection is with rca's then most propably a ground is touching signal. Have you checked both cables or just one? What happens if you rock the rca's at pre's output or power's input while both cables are connected? Unscrew top covers-units off mains- and check that rca's at both units have a tight fit.
You can use a voltmeter to check any kind of shorting if you wish.
G

At first I thought that the title of the thread was in a different language.

Have you tried interchanging the cables? I had a situation where one of my cables potentially had damaged shielding and would catch radio interference when connected to my turntable.

have you try a ferrite ring on power cord of preamp and or amp close to the unit

Nice one roxy54
All you can do is try every combo and it still might not go away. Move the pieces and IC around. I have a bag of iso transformers I try..
Start with in between.
Lotso luck.