Audi a6 avant 2002 2.0 petrol 100700 miles

Hi ! I have a hydroelectric vacuum pump to assist the brake booster unit. the pump has recently started to vibrate for about 5 seconds when i press or depress the brake pedal. i tried changing the pump with one from a breaker as new from audi are expensive, however the problem remained. the brakes dont seem to have been affected but every so often the power assist does not kick in making it very difficult to brake. this has only happened twice though. have i been unlucky and got another faulty one from the breaker or could there be another reason ? i have also checked all the hoses which seem to be ok. i have searched the web for an answer without success, your advice would be very much appreciated ! Gary :?