funky noises

[1980 911 sc]

I have owned my 1980 SC for a few months now. God is it fun!

anyway recently I started hearing a wierd noise. Kinda like a knock noise, but kinda like a chain sound too. it is only audible if I am in gear and put my foot on it more than half way. It idles and revs nicely, with no noise and it also pulls nicely when I do step on it.

it looks like it has the upgraded chain tensioners cause I can see the oil lines for them. could these be failing? what does it sound like when they do? maybe a bad chain guide?

I thought if it was a valvetrain issue it would make the noise all the time; is that true?

could I just need a tune up?

any help or ideas would be a welcome sight.

Thanks!

Mike