Under the heading of "at least it's cheap", they do make oil addtives to unstick and quiet noisy lifters. I had a 420 that when I let it sit for more than a week or so the lifters would misbehave. On start up it would knock and shake for a few minutes, and then it would be fine. It could be a stuck lifter or oil in the lifter leaking down. Then it fills up again and it's OK. At the time my mechanic recommended the additive, which I put in, but I can't really say if it did any good because I decided never to let it sit for a week. Another thought - could be an oil flow problem - clogged oil tube or something like that. It would be good to figure out where it's coming from.