Unwanted pulse sound from monitor amp

[Danelectron]

I have just begun recording using Sonor and will have many questions, but for now I just have one or two.
I am using...
Intel PIII 450 with 320m RAM, 13 G Hard drive
Creative sound blaster PCI Sound card
Delta44 Sound card
ART Tube MP
Shure sm57
AMT-1 Matching Transformer

Marshall 100Watt Head W/4X12 cab
15 watt Crate practice amp (for monitoring 'til I get some powered monitors)

My main question is that I hear a pulsing click from the monitor source (the little Crate). I can playback a recording through the Sound blaster card to my computer speakers just fine(no Pulse sound), but I get this unwanted sound from the Crate when it's plugged into the Delta. Even when the computer is shut down. It's not present when I unplug the cable from the Delta to the Crate.
Is this some kind of weird harmonic coming through the power cable to my pc?
My recordings sound fine other than this. I'm just concerned that when I get some powered monitors (I have the new YMSP1s in mind) this pulse will still be present.
If it is a power source (public utility) problem, would an isolation transformer be the answer?

My last question for now is, when I bought the ART the salesman suggested that I could use a matching transformer (AMT-1) with the Shure mic instead. Would this work well for micing an acoustic guitar or did I throw away the $15 I paid for it. I bought it for this purpose without thinking about the advantage of using the ART instead (tube warmth).

Thanks for any help/suggestions.

Dan
P.S. I got a good deal on a PIII 1000 that I will be setting up exclusivly for recording.

 

[Danelectron]

I feel so alone.
Could this be an uncharted phenomeonon?

I may run an extension cord from the neighbors house to eliminate any oscillations that may be coming from my circuits.
Then I guess I'll try moving East of the Rockies. They says clean power runs through the streets like a spring rain over there. And it's affordable!

Thanks.

Dan