Can almost guarantee the pickup screen on the oil pump has fallen out of the oil pump....adding more oil brings the sump oil level up over the inlet hole and oil pressure comes good again.
One cylinder noisy could also be a bad lifter leaking down and opening up the vale lash setting...this is what you hear as noise...
When its noisy, remove the valve cover where the noise is coming from and check the valve lash on each valve....just turn the nut in middle of each rocker down 1/4 turn while engine running...the noisy one will quiet down immediately.
45 going down the road on a dash gauge seems low...can you check the oil pressure with a different gauge?
Let us know how you go...
