Some 300 miles after changing the filter and oil (Castrol) in my late 2000 Rover 25 (mileage 40,000) a very light tapping sound started in the rocker box area. I assumed it was a piece of dirt in the hydraulic tappets so drained the system before replacing the filter and (same) oil in the hope the start up pressure of the oil would dislodge the grit. No luck. Now some 50 miles later it is a fairly distinct tap. Must I strip and clean the tappets ASAP or can I leave it for a while to see if it clears without causing any serious damage?