It could be one of the hydraulic valve lifters being worn. Our engines are zero valve clearance design -- any clearance is taken out by the self-adjusting hydraulic valve lifters. These lifters work on engine oil and when they wear they can gradually lose the hold of oil and fail to take out the valve clearance, or part of the clearance, hence the noise. Worn lifters are usually sensitive to engine oil pressure and viscosity, so top up your oil to a bit over max. mark and see if anything changes.
I used to own an old Nissan 1990 Stanza that had a badly worn valve lifter on an exaust valve. It caused significant valve timing lag whenever oil level is 1/3 below the "Hi" mark, and the engine would give off a "puff...puff..." exaust noise when oil level bottomed, probably due to the severely lagged valve timing and insufficient valve opening. I did nothing but kept the engine oil topped, and that worked for me.
Hope it helps.
