weird noise while driving through heavy rain

During heavy rains when there is 1-2 inch water on the road, my PA (91) begins to make a weird noise. It is very loud, the first time I thought the exhaust must have broke off or something. It is a resonating, rumbling low sound which changes pitch with revs and speed.
I figured it must be the serpentine belt slipping, as the sound stops shortly after you get to dry road.
But today I coincidentally parked the car and revved the engine, and there was nothing out of the ordinary.
What could it be? Note this only happens during heavy rain with 1 inch of rain on the road, never during "normal" wet roads..:confused:

Next time you experience this noise. Shift it down 1 or 2 gears and hold the same speed. If the noise changes, it's engine related.
If it's the same, maybe it's the tires.

During heavy rains when there is 1-2 inch water on the road, my PA (91) begins to make a weird noise. It is very loud, the first time I thought the exhaust must have broke off or something. It is a resonating, rumbling low sound which changes pitch with revs and speed.
I figured it must be the serpentine belt slipping, as the sound stops shortly after you get to dry road.
But today I coincidentally parked the car and revved the engine, and there was nothing out of the ordinary.
What could it be? Note this only happens during heavy rain with 1 inch of rain on the road, never during "normal" wet roads..:confused:

Its belt slip... Next time you get a new belt use the Goodyear Gatorback belt... It does a good job for wet weather driving.. I have changed all of my cars to this belt due to the nasty rains we get...

Thanks, do you have a part number for that ? The Goodyear parts finder doesn't know the ACDelco #.

Check AutoZone for the Goodyear Gatorback belt to fit your engine.

Thanks. It gives 2 Gatorbacks for the 91 base: the "Poly V (102,5")" and the "Poly-V alt & P/S (60,5")" but my PA only has 1 belt?
Also I've read somewhere that I should replace the idler and tensioner pulleys as well while changing the belt (is it correct that they are the same part according to Autozone?)

(edit) I just checked and my PA does not have an idler pulley ! Also the parts store here says the pulley on the tensioner can't be replaced, though I see it held in place by a 15mm? bolt.