chatter in power steering and brakes

[idahodiver]

at a stop my power steering pump vibrates when I turn the steering wheel. also if i step on the brakes firmly at a stop sign the same vibration is made... kind of like the abs kicking in. is there something wrong with the vacuum pump or anything that connects the 2 and would cause this. when i am going more then 20 miles per hour or so there is not vibration in the steering unless turning very sharply