Tenant complaining about ticking copper pipes

[Marc-Anthony Signorino]

This is in Cambridge, MA; five-unit building. We've recently updated the heating system in the building with a high-efficiency gas-fired water boiler getting away from oil. Because it's an older house, it's got copper pipes. One of the tenants (and only one) is complaining that he can hear 'snapping' coming from the pipes at all hours of the day (so he says)- which is likely from the plumbing expanding and contracting. I had two different plumber try to re-create the problem, but no dice. One said he can replace the copper pipes with pex for $2K. I think this guy might be super-sensitive to this - stuck at home because of COVID, going stir crazy, etc. - and by how he communicates, may even have some mental health issues.

How should I handle this? If I spring for the plumbing fix (and that's only in his unit), there's no guarantee it fixes the problem, it creates a bad precedent for him and other tenants, and who knows what you'll find when you open the walls. Plus, it's a whole month's rent blown. I can offer to buy him a white noise machine or noise cancelling headphones, or to move to another unit. If he decided to sue (presumably for violating his quiet enjoyment), how likely is a Mass. court to support a claim like this? Thanks!