very common:

it's a rubber vacuum hose having rubbed
against the steering column- just above the brake pedal- behind the lower dash panel. The vacuum hose supplies vacuum to the left rear corner of the trunk, next to the tail light unit , for the anti noise
flapper valve seen inside your right tailpipe. At different throttle settings, different RPM, and engine load, vacuum flows (or now leaks and makes noise like a balloon when you make it sing by stretching the air inlet) to the flapper valve reducing exhaust noise - but not performance.

Remove the lower trim panel and cut out the 2 inch section of
chaffed hose and insert a hard plastic repair splice in it's
place. Tie wrap the hose away from the steering column.
You may have been hearing an abnormal rattling noise from under , or rear of the car - more so during motor warm up as a result of this vacuum leak. The rear flapper valve will rattle because of low vacuum supply.

Bob's Yer Uncle .

PS Worked for me................