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!
