this sounds like it could be a crank big-end shell thats worn and I'd 1st suggest to measure the actual oil pressure with separate mechanical gauge - this can be plumbed into the engine via the current pressure switch port - if you use a Tee piece you can keep the pressure switch plumbed in as well and so see if the low pressure light corresponds to a low pressure reading.
If you do have a low pressure reading and to turn on the light it will be very low about 5psi then replace the oil pump and ensure that the pump pickup in the sump hasn't come loose or is blocked
if the noise is a light tapping from the cam cover then yes its likley to be a sticking tappet
and beofre just replaicng the oil then I'd do an oil flush treatment as this can help loosen any dirt and deposits that prevent the follower from pumping up correctly
do you still need help?
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