I really think what you're hearing is just the chemistry being sucked off the bottom of the front tanks as the volume gets lower. As long as you can run prints through it, the pump is probably fine. If you run it with the lid open, you'll probably see what I'm talking about.
You could take a peek under the machine--see if you can spot any leaks from the pipes--crusty, wet etc. It could also be that the standpipes aren't sealed properly, or maybe the o-rings are shot--so the chemistry could be leaking out slowly. I think if you can still use it--get prints through it--it's probably okay. I think if the pump died, you wouldn't hear that noise, but I'm not in your shoes....
For chemistry..... The tank solution is like 15 liters or so. The chem kits from Ilford are 3 liters, the machine dilutes it to 1:4. Polymax RT would probably work. I know Agfa Multicontrast would work at 1:4. 2150XL used to be sold in a larger container (less concentrated) that the machine diluted at 1:2.8 or something odd like that. I think all sorts of chemistry would work in it, once you got the ratios figure out--but replenishing would be a total PIA, so I don't know how much throughput you'd get.
There's this company called C-Pac that sells direct--makes chemistry for labs. They sell a 2150 kit:
called RC-XL. we almost got some of this recently because our Ilford order was held up & we came close to running out of chemistry.....it came through though--got a years worth of chemistry and it was in the new packaging so I think it's still being made. I'm more worried about spare parts and tech support than the chemistry to be honest.
Good luck though--KT