2004 Volvo S60 squeals in the rain

when driving in heavy rain i get an on godly high pitch squeel from the muffler area, sounds like a tornado siren and i have to stop the car as its to loud to drive

Are you certain it isn’t coming from the engine compartment? Have you tried to get the whistle while listening under the hood?
Your symptoms are often caused by a tired/stretched accessory belt or a tired belt tensioner… or both.

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Are you taking about the exhaust system in general or just the muffler that is at the rear of the car?
If its the muffler, it could be on its way out.To test: start the engine and make sure the gear selector is in “PARK” , plug the tailpipe with a rag and look underneath for any escaping fumes and unusual sound.