HVAC copper line whistling like crazy sometimes.

[Chicago]

For a while now there has been a high pitch whistling noise at random times but usually at night. Sounds like a police siren/slide whistle/wailing. Wakes us up. I thought it was an air leak somewhere in the walls but turns out it’s coming from this copper pipe which I think leads to the AC unit outside. When I hold the pipe I can feel something move when it’s whistling. Anyone know what it is and how to fix it? (I don’t believe it’s leaking. The AC works fine. The whistling has been going on over a year.). This noise occurs when the furnace is not on. Not sure if it occurs when the AC is on. Does also occur when the heat/blower is on.