Does anything move in middle/rear of X5 when in Park and idling???

[khelms_98]

The other day, I was standing outside of my parked X5 talking to a friend with the engine idling and I noticed a slight pop/knock type of noise coming from the middle or rear-end of my X5. Since the vehicle was in Park, I don't think anything is moving/turning in that part of the vehicle that would cause such a noise. In saying that, I think it was just the heat of the running engine with the extremely hot southern atmosphere that was causing the exhaust pipe or something to expand and make a slight deep popping noise that I first thought was a knock (almost like a hollow barrel being thumped). It's something most wouldn't even notice and certainly wouldn't question but I'm very very picky and I like to chase down every little noise. Almost like OCD. So does anything move in the middle/rear part of the vehicle when parked that would cause this noise, or do you guys just think it was metallic expansion? If anyone has any input or knows what I'm talking about, please let me know. Thanks