If they are the traditional pushrod type hydraulic "lifters", it's worth taking them out sometime (if that's practical due to the way it's assembled) and depressing the check valve with a thin piece of wire down the hole, and pumping them a few times to flush the working chamber. It can be a little bit of an art to do this, you need to hold the valve depressed while pushing the plunger up/down. They're usually a ball valve, sometimes a flat disc valve, and it's quite possible to get debris under the seat. Ideally this is done immersed in something like white spirit so it flushes the oil out ef