Eight months ago I replaced all four differential bushings because I heard a knock somewhere under the car when I was moving the car (from stationary, starting to accelerate). The mechanic found one bush (Rear diff mount - front bush) actually leaking.

I asked Lexus for replacement parts but they don't supply them, so I went for the polyurethane ones.

My question is regarding the two front bushes marked in red below:





Now on the original bushings there are these caps on top:



I've watched this video on YouTube ( ) where they replace differential bushings in IS300 and at 26:08 they're saying you have to put these caps back.



The only difference is, at around 5-6 minute they say they're using poly but they're solid and they don't come with caps/washers like the ones that I've bought:





My mechanic fitted these without the original caps because he said the new bushes came with washers. Is that ok or should these front bushes have this washer and then the original cap on top of them?

I'm asking because I started hearing a knock again somewhere under the car (most likely in the rear) usually after breaking harder to stop the car, for example when suddenly the traffic light changes from green to red and you have to stop the car quicker.

Thanks in advance.