If it doesn't burn oil and the noise you describe goes away or gets less noticeable once the engine gets up to temp, the chances of the noise being a lower end knock is very small, this leaves the valve train. If lash hasn't been adjusted in a long time the noise you are probably hearing is excessive clearance (lash) between the valve stem and the rockers for the intake valves.
Intake lash tends to loosen over time from wear creating noise especially on cold engines before everything has grown due to heat.
Exhaust lash tends to TIGHTEN over time due valve seat erosion and receding further into the head due to the inherent heat.
Excessive valve lash on the intake just makes for low power and a noisy engine, however, lack of lash on the exhaust will eventually lead to burnt valves and loss of compression. I highly recommend adjusting your valve lash first, a different weight or brand of oil may possibly quiet things, but is only a band aid.