my car has a pulse

[chris1212]

my 96 240 has developed a pulse at idle just sitting. it will almost cut off just sitting. the car drives fine with no problems at all.im also hearing a humming noise. it sounds like it may be the alternator or the water pump.but while sitting in the car it sounds like its coming from the rear.guess its time to pull this old engine and put in the built engine that i've had for 5 years and never turbo'ed