It's really hard to evaluate using a recorded media like that, but I don't like the sound of it either. The frequency is too high. I can deal with a "tap" at idle, but it's almost a "buzz". I'm thinking alternator bearings or brushes?

Or, it could just be impossible to tell from a recording like that. 

I get a distinct sense of rocker arms tapping on cam lobes... who knows.

Here's a sound for comparison - 

 

I have a Bose system, bass speaker with 2 surround speakers. Sound is wonderful.

I'm curious, have you reversed the two connectors mentioned above yet?

Thanks for the replies everyone.

Haven’t had a chance to get to the old girl this week as it’s out at my lockup (problem in London having no garage!), will have a tinker this weekend on all the above possibilities and get back to you. Really appreciate the help and will be in touch over the weekend. 

JD

When you get this problem solved, replace the Champion plugs with NGK's. You will notice the difference.

Edited May 25, 20187 yr by EuroDat