The frequency switch is instant - we go from one freq to the next in much less than 10ms (it needs to do it this fast for RDS functionality to be "invisible" (audio-wise) to the user.

I suspect that the DSP needs to be muted until everything is ready, though. You're probably hearing a totally different frequency (possibly 98.5MHz, which is a local station here that the tuner defaults to) just before your chosen station.

Hugo