When I start my car it's fine but once it fully warms up after a half hour or so there's no oil pressure at idle. the warmer it gets the less pressure there is overall until finally now my motor knocks. I know it isn't the sending unit because it's new. I'm wondering if this could be an oil pump problem or bearings or what. obviously the main/rod bearings need replaced now. has anybody had this happen to them. what do you think about the pump.