GM 3.1 V6 cold knocking

I have a '92 Pontiac Trans Sport SE with the GM 3.1 V6 engine and 91.000 miles on the clock.

My problem started as a discrete knock on a cold morning and developed into a loud knocking from the engine bay everytime I first start the engine. I have recently changed the oil with a good synthetic but the knocking stayed.

Problem is that once the car is running for a few minutes and the engine gets to operating temperature the knocking dissapears and things sound as normal as they can, even at high rpm's.

Does anyone has a clue what could cause that knocking sound on cold running and what can I do to fix it?

thank you!

2 options, generally:
1st is a lazy/leaking hydraulic valve lifter that empties completely when engine is stopped then fills up slowly with pressure after engine startup.
2nd is piston slap...a common occurrence plaguing the 3.1 due to piston skirt erosion.

IN both cases, it is an annoyance and has no real effect, neither instantaneous or long term on engine performance. ....but it is annoying.

Other possibilities, though remote, would be oil pressure, but that would rather be the opposite, that is, noise would increase as engine gets warmer....Problem is you do not know the oil pressure because the 3.1 has a fake oil pressure indicator, just indicating a fix pressure as soon as you have more than a few psi....

In this case.. would an oil additive work in any way to reduce the noise when engine is cold?

This might sound too simple, but what brand of oil filter is installed? Cheap ones like Fram allow oil to drain out when shut off, resulting in a minor oil starvation problem.

It could be a cheap filter - the only one I could find here (Romania - Europe) for this type of car was a "Wix" filter which costs around 5$ in US currency.

Can you recommend a better filter - I would try to order it online and install it.

The options I see now on rockauto.com are:
- ACDELCO DURAGUARD for 4.24$
- WIX (like I have) 4.15$
- FRAM (I understood this is a no-no)
- K&N 8.14$ (do you know anything about this brand?)
- BOSCH 3.99$ (I know this brand but could it be any good at this price?)

What would you choose for this 3.1 GM engine?

Well, WIX filters are about the best out there, other than AC Delco for your van. In fact using a WIX filter increased the oil pressure in my '93 Trans Sport from about 18 psi to 30 psi at 60 mph over the cheap Fram filter, so its not likely to be that.