Wild idea:


Maybe drain out all of the oil. Replace (on a temporary basis) with a thinner oil. It might help force oil into the lifter???


I wouldn't leave the thin oil in there in any case. Especially for summer driving.

 

I'm not denying that there are never any lifter related failures. But, syn oils because of their molecular structure cause lifters to bleed down sooner and at times take longer to pump up. I'd give give it a good drive of 30 minutes or more and don't be afraid to let the motor sing along at 4K RPM for a bit, of course after op temp is reached.

My wife's Boxster which is stored for most of the winter does this every year. Has since we've owned it for nearly 12 years. Once the rattle has subsided, it's quite till the next end of storage period. If the noise does not diminish after a hundred or so miles, change the oil and filter, drive more. Still noisy, got a bad lifter.

 

I remember when I rebuilt the motor in my eagle talon that I drag raced. I started it up for the first time (hydraulic lifters) and the motor was banging so badly honestly I thought it spun a bearing. It banged and banged for 15 minutes solid, seriously I was ready to pull the motor out and cry. (I was 20 and spend all my saved money on the rebuild / perf built)

It started to quiet down a bit after 15 mins, I took it out and drive it for another 15 mins light, came back and it was quiet and totally normal sounding. I just took 15-20 mins for the motor to warm properly and hydrauil lifters to pump up. Was very surprised how long it took. Motor ran for years at 425whp level with no issues at all. 24psi on a 4 banger after all that noise, was as reliable as a wood burning stove.

I suspect you need to warm it up and drive it considering it was fine last year.