You can make a 210 whistle by closing the right or left side dash vents. But that's a continuous whistle rather than occasional, and doesn't matter whether a/c is engaged or not.

If it only happens when the a/c is on, I'm betting the source is the expansion valve, and the cause is low refrigerant charge. All my 124 cars became very talkative if the refrigerant was low. An expansion valve whistle cycles on and off, lasts for a few seconds, and is more likely to happen when accelerating because the engine is at higher RPM.

I suggest you check the refrigerant charge for starters. Might be an easy fix.

- JimY