Rattle Under Valve Cover

[pcfixer]

Hi, my Lexus RX300 has 180k miles, timing belt replaced, last month while I was driving I heard rattling under the valve cover.  It starts after driving a few blocks when I let up on the accelerator while coasting.  If I step on the accelerator the noise stops but as soon as I let up it starts sometimes very load. If I let it sit in the driveway the noise starts and gets very load after a while.  I took it to a local mechanic and he said, "beats me" lol.  I replaced the bank 2 OCV and cleaned the filter with no change.  Anyone has any ideas?  It would be greatly appreciated! 

[Drcoffee]

remove the valve cover and see if a bolt is bouncing around in there.  As noted below, could be valve lifters.  These engine are prone to sludge.  I certainly would not ignore it.  Have you checked your oil level lately?

[Taffy13]

I would get the Valves clearance  check and  adjust that's my opinion.