My new to me '91 300 SEL has a humming noise coming from the rear. Low pitched. Guessing it might be the fuel pump. Is that a usual noise? Nothing wrong with fuel supply. Sure it isn't tire hum. More 'electric'. My old MG's had a fuel pump sound but was a 'click' as it was a true pump. Any ideas? ps- you sure helped me change the light in the shift boot. Great directions. You guys know your stuff!