I have a 1999 RX300. 260,000 miles, second transmission, timing belt done a few years back. I don't it that much, but I'm keeping it and putting money back into it.

On a cold start, I'm getting a very loud knocking sound on the top of the engine. It lessens as the engine warms up, but doesn't completely go away. Thanks. PM260k

 

Since no one answered ,..

Check engine oil level.

For diagnosis you will have to take it to a mechanic for a hand-on inspection and even a check with mechanics stethoscope to narrow down the source of the problem. From the net you will get wild theories, one as good as another, but you need a hands on checking.

Salim

 

Loud cold start noises are normally related to slapping belt, chain or tensioner..
You may also have it from VVT-I actuators, as they are oil pressure driven. Or, simply from rockers, if tappets are worn out and have to be replaced to adjust gaps.