1988 Jaguar XJ6 ENGINE NOISE

I have an 88 Jaguar XJ6, it was running fine when I put it away for the winter. I started it up today, and took it for a drive. I noticed there was a engine noise when I accelerate, it actually sounds like a knock in the engine, but only at high rpm's. I checked the oil it is fine, and clean.
I don't like to put additives in my oil, but is there something I should try? Or is it just because it's been sitting for a few months.

THANKS!