Just did a 300 mile trip, San Jose to Santa Barbara on 101, and learned
that the noise level from heavy rain impacting the roof is REALLY LOUD.
Seems to come mostly from the front section of the roof, pretty much
independent of speed. Combine it with the usual tire and wiper noise
associated with a wet roadway, and you can forget about listening to
that nice XM radio, or even much in the way of conversation. Some
serious padding or insulation needed up there!