Expansion Tank sounds like a foghorn

[FlyersFan]

We recently dropped the house water pressure from 70 to 60 psi and increased the pressure in the expansion tank from 40 to 60 psi as I was told that they should be the same. The hot water tank and the expansion tank are 3 months old and there were no issues when the house pressure was at 70 psi. There are no issues when we open up a bathroom or kitchen faucet. However, when we flush the toilet or turn on the shower the expansion tank sounds like a foghorn for a few seconds. If the toilet is flushed 5 minutes later all is quiet. But after 15 minutes the foghorn sounds will return as the pressure in the line has time to build up. I have turned the water off to both toilets and the sound is still there when we turn on the shower. Everything I read online states that the noise could happen when the pressure is to high or if there is a large discrepancy between the house pressure and the pressure in the expansion tank. Any help or guidance would be appreciated. Thank you.