i notice sometimes when i am backing up i hear a sound its not a loud "thud" but it happens every so often any ideas? for example in my old suburban when on an incline (lauching a fishing boat down a ramp) with a load on when i put it from park into reverse it felt like a heavy amount force and a kind of thud occured. this sound i am hearing every so often kind of reminds me of that.

i remember with older trucks when you change the brakes you must reverse and press the brakes to calabrate them and if you did alot of backing up the brakes would act a bit funny

kinda thinkin out loud...

Sounds like the driveline, makes sure the joints and slip yoke are lubed up.