I am not sure of the answer to your question but I suspect over 20 years the blades of the fan will collect some muck and grime which will affect its performance. The air pump on our 2001 36C also makes more noise than before including the high pitched noise you describe. It varies with engine revs which I think is due to whether the alternator is pushing out 13v at tickover or 14+v at 2000rpm.
I had also thought at having a look at the pump but on our boat but keep putting it off as it is tucked away in the aft lazarette just under the deck and is a right so and so to get to. If yours is in a different location I would be interested to know where.
I think I read somewhere the pumps are rated to run for a 1000-1200 hours. As we have done over 1700 hours I suspect it is due for replacement.
Let us know how you get on.