Toyota - Camry :: Loud Banging / Grinding Noise Only During Heavy Downpour Rains

I have a 2000 Toyota Camry LE with 106K miles. It?s getting a little clunky but works perfectly. I do regular maintenance like oil changes every 3 or 4 months, breaks, tires. Last year one day during very have down pour rain I noticed something banging/hitting under the seats while driving in this heavy rain & car losing power (slowing down). Something (I am guessing the exhaust pipe? hitting the body. The more gas I gave to gain speed the violent the banging. I slowed down to almost 15MPH but never stop & the car started to work normal. It happened 3 or 4 times before I got home. I check all fluids etc? All normal. Drove later in sunny, cloudy & even in light rain the car runs perfectly fine. Then it happened again in heavy rain.

I told my car mechanic & he checked all even tightened the main belts (not timing belt). No change. Now it?s so predictable when ever its rains heavy & even slushy snow/rain. Lately it started to not only lose power but sometime instead of banging I hear grinding noise when pressing the gas paddle. Some time banging followed by grinding but every time after slowing down the car recoups power and I drive normally. It never ever happens in dry day. My wife who drives ?Buick Century 2004? thinks my car is dyeing and should buy a new. When I describe this situation to any mechanic they points to transmission & want to perform brain surgery to diagnose.