Okay, I swapped slots between the Delta 66 and my Ethernet card. My
machine wouldn't boot, no idea why, other than it's a crappy machine
that sometimes does stuff like that. So I pulled out the Delta (it's
going in the new machine anyway) and put the Ethernet card back in its
former slot. The machine booted, and 'cat /proc/interrupts' confirmed
that the soundcard (an SBLive Value) is on IRQ5 and the USB ports are on
IRQ 3. The noise from the printer is still present, but not so loud now.
Also, there's no added noise when the printer is working. Formerly it
made even more noise while printing.
Eventually I'd like to add this printer to my new box. I'm curious to
hear what happens then...
As always, muchas gracias to everyone who replied to the topic.