Are you saying that the pop sound appears only when you play red book cd and change to SACD? As we know, the D90 screen will display PCM (red book cd) and DSD (sacd).
When I play red book cd it will display PCM on D90 screen. No issue with red book cd, blu ray or DVD Audio discs. All are PCM. But once I change to SACD, the pop sound manifest at the very start of the 1st track. Thereafter, everything is fine. It appears again when I pop in another SACD or if I stop and restart the same SACD, it pops.
Yes, when the D90 is switching from PCM to DSD. If I play SACD through it one after another, no pop. So I make sure I turn the volume down before I start playing an SACD.