Abe, I had the same problem with a Zoom 505. I am playing through a '65 Fender Super Reverb. I always had a steady hiss when the Zoom was plugged in, even in bypass. I exchanged the pedal and the problem continued. The hiss disappeared when plugged into a solid-state amp or new-design tube amp.

What we finally figured out - the Super Reverb has a very "hot" (high-gain) preamp with a large dynamic amplification range that amplifies the noise. I suspect that your Marshall is similar.

I tried connecting to the #2 jack (which has lower gain) and even replacing the 12AX7 tubes with lower-gain 12AT7 tubes. Both of these reduced the noise, but did not remove it. I adjusted the output gain of the Zoom and was able to "tune" the pedal for less noise, but it was still too much in the environment which I play. These changes also changed the way that my amp sounds - I finally sold the Zoom and carried my entire rig with me when I went to buy pedals.

I ended up with a Danelectro Chorus and Delay (very quiet) and a reissue Ibanez Tube Screamer (quiet enough). Recently I have added a Yamaha DG-Stomp with a Morley A-B-Y switch for bypass. The DG-Stomp is very quiet - it has no noise gate; it doesn't need one. I play either a '68 Les Paul Custom or a '98 American Deluxe Strat (with the "noiseless" pickups).