1994 Mercury Marquis Rear End Throbbing Noise

Getting a rear end throbbing noise (like running over rumble strips) intermittently around 30 - 50 mph. Also a intermittent high pitched whistle from the engine. Not sure symptoms are related as just bought car used. When throbbing noise occurs in rear end the car engine has a hesitation. could the abs be engaging one of the rear brake drums? some recall wit the speed sensor/cruise control? thought the high whistle might be the serpentine belt on a stuck pulley or an about to die water pump bearing or some air mass/vacuum leak. Just guessing stupidly as you can tell. Love the car and all else works great. What to do???