A few weeks ago our bathroom sink drain started gurgling. It happens
about 30-90 seconds after you stop running the water at the sink. It
wasn't happening previously and the house was built in the mid 1960s.
The gurgling also seems to be only caused by running water at that sink,
flushing the toilet or running the shower doesn't seem to cause it.

Unless a flying squirrel jumped down the roof pipe, I don't think
anything has happened to the plumbing vent. Also, the sink has been
draining ok, but maybe not as quickly as it used to. We have copper
drain pipes and I'm not sure if drain openers like Drano (NaOH) will
corrode the copper.

Any ideas? Thanks!