Rattling vent cap......

Has anyone had the vent pipe under their kitchen sink rattle?  I reached behind the sink and was able to remove the cap by unscrewing it. There is a rubber flapper with a plastic piece and spring in the center.
If you tap on it you can hear something is loose but the piece is sealed.
Has anyone replaced theirs for the same reason?

Thanks

Greetings David;

The problem you are seeing is with a plumbing  fitting called a Studor Valve..[aka AAV]
it's used instead of a vent to the out side...
There is a spring in it, and it could rattle...
Depending on the make or model of the valve it could unscrew or disassemble..
It's important to keep sewer gases out of the cabin..
You can find them at Home Depot for less than $10...
Take Care
Tony