I went to quite a few garages, and they could not find any fault with the suspensions. The O/S front CV joints are shot, need to replace bushes and front upper arm wishbone had it's fair share. Even then the noise is coming from the rear, I can clearly hear it.
If it's not the plates causing the problem, then what is?
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It may be the plates but a "knock" doesn't make sense as they are wafer thin steel. To confirm cut them off and see if the knock is still there as hub removal is a PITA and may not be necessary. If something was dangerously amiss with the suspension then the garages would have identified it so all you have is an annoying sound.
Roll bar knocks are certainly difficult to diagnose on a stationary car even with access to a ramp. Could be this. You may be able to see evidence of wear the roll bar is touching somewhere.
Have you given the exhaust a good shake if it touches the underside of the car then this makes a very loud and unnerving knock.
Look in the boot. Is there a lose wheel nut in the tyre well? Is the spare secure?
Does the knock present itself on bumps, when cornering or braking, at certain speeds? Is it on one side? Can it be felt through the floor? Has it had any work on the rear done by previous owners?
Basically, I think you can assume it is safe as more than one garage has investigated. It's up to you to decide if you want to throw money at it. To put things into context recently i spent £800 replacing rear suspension components. Not fun.