My friend has a beautiful manual 1974 series II with 127 k miles .
It gradually has developed a loud ticking noise near the top of the engine,also existing when warm.
Performance is good.
Several garages have tried to diagnose the noise...
In my opinion it could be a loose tappet guide.
Will it benefit from a stakedown kit? ( i have not tried to feel it through the oil filler inlet yet).
Your opinion on the possible causes of the noise in the video?
Many thanks in advance for your suggestions!

It could well be a loose tappet guide, so to avoid further damage, your friend needs to remove the cam covers for an inspection. If a guide is loose, thenm yes, a stakedown kit will cure it. I never had a loos guide on my 1980 so I've never installed a stake-down, but others on here have, so hopefully will guide you. If it's not a guide, check all valve clearances, it might be a bent valve due to it being a bit "lazy" coming back. This is something I did have and its a real PITA as the head has to come off.

I have installed dozens of tappet holdown kits since the 1980s.
I used to install them on cars that had no problems (YET) but people wanted the 'peace-of-mind'.

Not hard to do but I have had to replace a few tappet guides when people went too long with the noise and cracked/split a guide.

We had a 'donor-head' in the shop that we pulled good tappets and installed them in heads with broken ones.

You can see if it has the stakedown kit just by opening the oil fill cap and looking in. The stakedown kit is somewhat obvious. And... if it isn't there, then yes, pull the cam covers. Installing the stakedown kit is really no big deal. Can easily be down with the cams all in place.

Thanks for your quick replies!
The owner of the car wil soon take it to a shop with (more) experience with the "older" Jags.
I will try to let you know the opinion/solution of this shop.
Regards, Jan