There was a thread a few days ago about Sunfire subs popping and humming.
I have had pops when the sub turns itself on and off. If your system has been off for a long time and the sub suddenly pops, this could be the sub turning itself on when it perceives a signal. Since your receiver is off, maybe something else like a power line spike is faking out the sub.
If your system has been playing recently, and you turned it off 10 or 20 minutes ago, then the pop could be the sub turning itself off. I observe with my subs that they pop themselves off if they sense no signal for a while.
Of course, while these are what I think might be pops associated with normal operation, you might have pops due to a malfunction.
I have had pops when the sub turns itself on and off. If your system has been off for a long time and the sub suddenly pops, this could be the sub turning itself on when it perceives a signal. Since your receiver is off, maybe something else like a power line spike is faking out the sub.
If your system has been playing recently, and you turned it off 10 or 20 minutes ago, then the pop could be the sub turning itself off. I observe with my subs that they pop themselves off if they sense no signal for a while.
Of course, while these are what I think might be pops associated with normal operation, you might have pops due to a malfunction.