mine pops when opening too, especially after its' rained or has been washed. I think this friction between the large sheet and small window sheets is causing a much larget problem wit hmy roof now. I'm gonna disable the rear window from tilting somehow (probably just pull the motor's cable). Should solve problems.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S