If you think you have an oil pressure issue, you would likely notice something going on with the gauge at the cluster. Does your oil pressure fluctuate or ride at a low level when you're driving and or idling? It's normal to idle at 1.5 bar but if not pegged at 2k rpm full bar to have an issue.

If that gauge doesn't work, hook up a mechanical oil pressure gauge.

A rattle could be a worn guide or timing chain. At 300k, your due for guides and a chain. Before throwing away this motor, pull the valve cover and check the chain stretch. If it's over 4 degrees, time for a new chain and guides. You don't mention running it low on oil, so I highly doubt your bottom end is having any issues. The mechanics theory or diagnosis is absurd to me. I daily drive a half million mile turbo diesel on the original motor. A lot of mechanics are idiots when it comes to these cars. They think it's just some normal throwaway civic that blows up at 150k.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k