Firstly, have you swapped front and rear wheels to see if the sensation changes, that would be my first move.

Secondly if the brakes were knocking, which ones?, there lies the clue where the problem is, i'd be concentrating my efforts where the vibration was heaviest, can't rely on the fact the wheels still in their original places hence suggest rotating.

Assuming you can't pinpoint where the vibration is coming from, it sounds like the car needs to be raised and all wheels spun fast and look and listen carefully for things out of true or noisy, fairly easy to do at the rear if you ease the pads back a bit...might have to remove them depending on handbrake design.

*At the front end you have an assistant drive the car with the wheels raised and see whats happening, check for out of true drive shafts this way, can be done wheels on and off. Don't need to tell you that this practice needs great care and a securely supported vehicle, unless you are a competent car DIYer not at all advisable for the uninitiated*.

The rear tyre wear as noted by the garage could point to a warped wheel, though a competent tyre fitter should have seen that when the wheels were balanced, it could also point to worn suspension bushes or shockers or bent stub axle...but there's no guarantee that the tyres weren't previously rotated so that poor wear might be at the front.

On the other hand the new tyres might not have been balanced well, so maybe recheck balance before doing anything else.

Edited by gordonbennet on 09/09/2012 at 11:12