Remanufactured engine knock
 

So, I just bought a 1991 ford ranger XLT 2 wheel drive with a 2.3l 4cly engine. I've had it for about two months and it's driving well. I noticed a knock in the engine about three weeks ago. It sounds like it's coming from the back near the bottom. It doesn't usually start until it warms up. When I first heard it I did an oil change to see if it had any metal in it, which it did not. It was dirty though. The oil I used for that change was 5w-20 with a NAPA filter. It was still knocking away, so I took it up to an auto shop in town and they said it sounded like the oil pressure was low and to replace the oil with 15w-40 some engine restorer and a different filter. Well the noise went away but the oil pressure did not go up and sometimes I think I can still hear the noise. The truck is still driving fine though.
Am I just paranoid or is it something to worry about?
Thanks

Welcome to the forum

1991 Ranger doesn't have a real oil pressure gauge, it uses an oil pressure switch on the engine, on or off switch
Switch closes(on) when oil pressure is above 5psi, and gauge shows engine has oil pressure, any movement of the needle after that is from the alternator's voltage going up or down with RPMs, not a change in oil pressure
Just FYI

A knock is not a good thing, the thicker oil will help, keep using it
5w-20 was way to thin for the 2.3l SOHC Lima engine, 1974 thru 1996, new they used 10w-30, but 10w-40 or 15w-40 is fine to use on any older engines

Thanks, good to know. I had no clue about the oil gauge. I am quite the novice when it comes to engines. I have been told it could be the rear main bearing or the flywheel. I guess I'll take it back up town.
We've got the stuff to pull it, if it comes to that.Thanks again for the response.