The USB lead is removable and seems to be "soundly" constructed, if you'll excuse the pun..the noise in fact occurs with other cables of the same type. The PC, amplifier, monitors and printer were initially all connected via the same power strip, but I've tried the printer connected to another room to no avail.
The sound card back plate is screwed solidly to the chassis, but the problem is also evident if I touch the front speaker lead to the metal of the PC case...apart from the expected noises, the interference is transmitted clearly through to the speakers.
The strangest thing occured when I was testing with a "dead" monitor - the monitor was unplugged from the mains and the USB connection to the monitor hub unplugged from the PC. The noise remained but I realised the DVI cable was still connected - I removed it, and lo and behold the noise was gone. It seems strange that interference from a USB cable can work its way to a PC via a monitor DVI cable.
I genuinely think it's a problem with the printer as the noises occur no matter what combination of power outlets and USB sockets (hub, monitor, PC itself) I use.
The only USB cables with suppressors (I assume you mean the cylindrical things in line with the cable) on them I have appear to be the mini-USB B types, for digital cameras, MP3 players and the like. The printer uses the larger almost square type at the printer end, and I have no other such cables with a suppressor on. I'll try pick one of these up somewhere, as part of the process of elimination.