This is one of the strangest fault I have ever encountered.

'98 GS300 with 185,000 miles. The engine makes a tapping (almost a knocking) noise which is speed related in park and there on tickover. It is fairly loud and can be heard within the cabin. There are no warning lights, performance is unaffected and the exhaust and oil level/color are good.

It appears to be coming from the engine itself rather than anything driven by the serpentine belt. Removing the oil cap has no effect on the noise.

The odd thing is that this can occur for maybe a week at a time and then mysteriously disappear for a few weeks.

What can it be?

 

Take off your serpentine belt and run it for 30 seconds and see if it's still there.

 

Do you feel it in the gear shift selector? Mine started like that then disappered then came back worse. It was a cracked flex plate. We put it up on a lift and you can hear the noise coming from the inspection hole between the engine and transmission. When we turned the engine off and tapped the flex plate we could here the same noise