single hydraulic valve lifter noise

[azurite300E]

As of recent only one of the hydraulic lifters in my M103 engine has been much noisier then the rest, making a very distinct click-click sound. I had replaced all lifters about 4 years ago with new Febi ones because some where at the time loosing pressure while the engine was off, so I was getting a very noisy cold start if the engine had been off for more then a day. After the replacement all was quiet until now.

I have not been able to exactly pinpoint the noisy lifter, but by listening through the valve cover it seems it is one associated with the last three cylinders towards the rear of the engine (firewall), but I can't really be 100% sure which one in particular. Since I had saved the old lifters (even labeled them by their original position), I exchanged the ones of cylinders 4-5-6 (one cyl at a time) with the these in an attempt to find the noisy one, but so far I have not been successful, the noise continues without any change. I am planning to buy a mechanics stethoscope in order to check each lifter with the valve cover off and the engine running. I already did start the engine this way in hopes it would be easier to pinpoint the noisy one, but without the use of a stethoscope this is seemingly impossible, and it starts to get very messy very fast.

In the mean time I wanted to ask if there could be something else responsible for the clicking noise I hear. I assumed it would be a lifter because the noise clearly originates from under the valve cover. Is there anything else I should also check for? I have looked at the cam lobes which all seem o be fine, and all the ball cups that go between the lifter's rounded face and the valve stem are in place. BTW, I've seen these ball cups being sold as 3mm spec, and I wonder if one of mine are worn down and might need to be replaced because of added clearance between the lifter and valve.

Thanks.