You may want to consider switching to Delvac 1 5w40 next time around. It isn't too thin for these engines mine love it and I have 254k on mine. It is a long shot but it may help.
I just changed all my lifters over the winter because they were driving me nuts! One of the new ones is ticking.
I'm going to drive it for a bit and see if it goes away. Lucky for me the dealer threw in two new lifters for free!
