Vintage Synth Repair

My mother has a Wurlitzer electronic organ (circa 1970) whoose keys are 
making an extra squeaking or rubbing noise when they are pressed and I 
am wondering what sort of maintenance the organ might require to 
correct this. I hear the squeaking/rubbing when listening to her play 
while I am in the same room and find it most annoying when I make audio 
recordings as my microphones are picking this up as extra noise.

Thanks in advance for any suggestions.