I have an 01 Tacoma prerunner v6. 2wd. When at idle or no load on engine I get a small knocking noise, sounds like its coming from top of engine closest to driver side. When any load is on engine or I give the slightest touch on the gas the noise goes away. My truck has about 160k miles. And I am about to put 3000 into it ie arb locker, gears, and preventative maintenance items. I just want to make sure my truck is running solid before I throw money into toys. Or if this is something I should figure out first. I have checked valve clearance and all in spec. Noise does seem to increase with rpm I belive. And I think it might be getting louder but could be my imagination.

Any ideas on what it could be or diagnostic tips?
All help appreciated thanks!