Timing belt was replaced and now there is a noticeable valve noise at 1600-1900 RPM. No noise at idle. Once you hit 2000 RPM and above no noise. Power is good across RPM band.
Checked valve clearance, nothing too far out of spec, a couple a little tight, but on the edge of the tolerances, so .009-.011 is specified a couple are .009. Where .011-.013 is specified, a couple are .011. (in.)
Noise is present even in low load conditions on level ground, if you press the gas and get into 1600-1900 RPM range you get some noise.
Noise is gone or much reduced when engine is cold.
A recent basic tune up was done, plugs, wires, cap, rotor, clean TB, air filter.
Tension on the timing belt seems reasonable, perhaps a tad loose, but there is no spec for the deflection that I know of, and following the official service manual for loosening the adjuster nut and moving counter clockwise 3 teeth on the cam sprocket was done to no effect.
What could it be? Is it something to do with VTEC? at 1600 RPM? some connector or oil orifice malfunctioning?
Is it simple pre-ignition? The tune up seems solid and power is good there is no specific high-load or high temp knocking I can detect.
Should I try higher octane? There is no need for that in the 2.3L 4 Cyl right? The compression ratio just doesn't need it...
2000 Honda Accord stock, no modifications, 111000 miles.