Lexus 97 in rain

A weird problem with 97 Lexus. During heavy rain it gives rattling noise underneath (under driver’s seat). Sound like something loose in the exhaust system or trasmission. Car loose power but no warning light come on. When the rain in over and no more moist, the car runs perfect. The mechanics could not find any thing wrong. mechanically everything is perfectly in place. Any comments/suggestions? thanks…mcisoccer