2003 Ford Taurus 2003 taurus vibrating when a/c on and when

Hi guys, i originally posted a question her last year about Pepboys wanting to charge me 1200 to fix my a/c. Well i just replaced the compressor,evaporator and drier and it costed me about 1000 anyway. Now my car is making a vibrating noise from under the hood when i stop at a red light and the brake pedal vibrates and a roaring vibrating noise comes from the engine area. It doesn't do it when the a/c is off. Ocassionally i hear the sound when i am driving too. I took it to the same mechanic and thought it was the motor mounts but they said something was loose and they tightened it up and it was quiet for a couple of days now it is starting up again. Could it be the motor mounts or a vaccuum leak somewhere? I am at my wits end with this car and once i fix one thing something else breaks. I'd appreciate any help again guys.

Thanks,
Debbie