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We have a 2003 imported Toyota Vitz which has recently developed a knocking noise in the rear (probably the left) when it is driven over bumps. I have checked the boot thoroughly for loose objects and had the mechanic who services our cars have a look at it. He could find no obvious source of the noise in or around the rear suspension.

Can you suggest a possible cause or where would be the best place to take it to have further investigation done?

As you can appreciate, it is difficult to diagnose a noise without hearing it. However, if there are no obvious worn bushes or loose mounts etc, the noise is possibly a faulty shoch absorber. Other areas to check for if it doesn't seem to be in the suspension/shock absorber area are the exhaust mounts, the latch on the rear hatch or bolts attaching the rear bumper etc.
A loose road wheel can also cause a knocking noise, but this is more likely to be continuous while driving, not just a one-off knock going over a bump.