1959 house with 90% of the plumbing updated in the last 10 years. House is two stories, water comes in through basement goes through a sediment prefilter then a Pentair whole house filter, and a whole house lead filter. Water heaters is tankless. Bathroom in question is on second floor. House also has an irrigation system. Towards the end of the irrigation cycle the sink faucet in the upstairs bathroom starts to whistle loudly. Running both sides hot and cold for about 20 seconds gets it to stop. It doesn’t start back up. I am wondering if the irrigation is causing a slight drop in pressure that builds up and by the end of the cycle is pulling air back in the pipes and that upstairs faucet is the ‘leakiest’ on that top floor. The problem is worse than last year with the new filter system. It still happened most cycles last year but wasn’t as loud and was the shower in that bathroom not the sink. If I am right now how do I fix it? I have the irrigation set to run at only 50% when it cycles again to see if that changes the pitch.

I have had some jobs that will get a noise in one bathroom and it is caused by a weak seal in a Fluidmaster fill valve in a toilet in another bathroom. Changed the seal and solved the problem.

You don't have same scenario but worth checking by turning water off to toilet(s) one at a time etc. and see if it makes a difference.

Maybe way off base, but just a thought.

Good luck.