So within the past week or so my truck started making a popping sound. It only happens when parked, parking brake applied, and after 30sec. It'll do it two or three times, single pop each time. Sounds like from the rear and I can hear it in the cab. Felt it lightly a couple of the times. Absolutely no popping/ clicking in D or R at any speed and I've only noticed it after a cold morning commute (could just be cause quiet outside then) . Don't think it's a heat shield due to location. I do have pinion seal leakage but checked the fluid the other day and it was fine, I check it once every two weeks until I can fix it. No axle seal leakage that I can see. Checked all brake rotors heat for brake drag, nothing seemed excessive. I have had to adjust parking brake before but recently it's had a hard time holding again on even mild inclines. I would think delaminated shoes would make noise all the time though, along with differential. Any other ideas? Suspension has been more noisy and rattly due to age and rough roads, so I bounced it a few times after I stopped and couldn't duplicate. Could it be bushings....shocks maybe....idk? She has 85k on her. Thanks ahead of time.