Mystery engine noise, hyd. lifters? Help guys...

[briscuba]

Hi, factory 402BB/400TH, headers, 1970 CST/400

I posted this about a year ago and have some follow up. STILL a mystery to me, hoping someone can help please.

I am hearing a "ticking" noise in my engine compartment that I'd say sounds just like a sticky lifter. We've all heard this ticking before with hydraulic lifters... However, I recently replaced all my lifters and found a bad one (did a happy dance thinking I had it licked!) I did not change the cam. Valve adjustment done many times, timed well and sounds great. Runs great. STILL ticking. Ugh!! So annoying.
Here are some details:
1. quiet with covers off, adjusted fine, all 16 valves. quite with covers on.
2. increase ticking on acceleration, decrease on deceleration.
3. lifters new, exh. manifold gaskets new, header to muffler gasket new
4. sometimes makes no noise at all
5. hot or cold same symptoms

See pic, I've been told the low profile valve covers sometimes have clearence problems? Don't think it's speedo cable... Any ideas?

Thanks guys!!