Yesterday I noticed a low volume steady tone while driving. The tone pitch stayed steady no matter what speed I was driving or what rpm the engine was running. The tone did not change when the car was halted and the engine was running. It seemed to be coming from the rear of the car and was loudest near either rear wheel well. It stopped when the engine was turned off. The car starts and runs normally other than the tone. No indications of problems on the idiot lights. My guess would be the fuel pump. Any ideas would be appreciated. Thanks.