Valve collet or idler pulley

About two months after putting my engine back in after a-full rebuild and using an AMC head it started to be more rattly than usual, enough for other people to comment. The thing is as it hasnt seemed to effect performance over the last year I dismissed it as probably being a rattly idler pulley. However yesterday another TD5 driver said he thought I had a failed or not properly seated valve collet as he had experienced this problem. This might make sense. I checked with the guy who rebuilt the engine and he said its possible but he had never heard of a collet failing. My local garage who does the big jobs on this truck said he thought it was an idler when it first started. Replacing both idlers will only cost about £110 but a collet is less than £2 but I think it would need to be done by the garage as Im not competent enough to make sure the cam and rockers are optimally set up afterwards unless its possible to compress a valve spring when its on the resting stroke (closed) and replace a collet?
Anyone had this problem or anyone able to advise me about replacing the collet as both RAVE and Haynes list a full strip down of the head and I am too simple to pull out just the relevant instructions

Put up a video mate of the noise in question, what oil are you using in the engine?

Thanks @Discodevon im using 5w30 fully synthetic. The engine was a complete rebuild and the head was new (not recon) so I am using the recommended viscosity not something thicker. Do you think that might be the answer

Sorry some reason video did not upload any idea how to do it from an Ipad?

It should upload if you select upload a file mate

Nope doesnt like the file type. Will mail it to my PC and try again

Ah its a Mov file and its huge. I will film it again on my phone

@Discodevon ive got the video in MP4 which is widely used but the forum doesnt include it on the file formats acceptable for upload. Can I message it to you?