Electrical, popping noise

[Stringer]

My 72 has a '90 350 TBI, 700R in it. Runs pretty good. I'm embarrassed to say that, for the last 6 months or so, it's making a popping noise...sorta' like little firecrackers and it's coming from the distributor or thereabouts. I'm embarrassed cause I haven't taken time to look into it until now! Runs fine - only makes the noise when it's hot. Audible in the cab...standing beside it.

New spark plug wires. New cap and rotor. If I'm testing correctly, my ignition coil is bad. Would that cause such a noise? Also, when I finally changed the cap/rotor this weekend (and they didn't look THAT bad), there's fine surface rust under the cap. I don't see any evidence of arcing, there.

As I always say in my posts, I'm a wannabe mechanic. I'm trying to learn, though. Any thoughts?

[68panelman]

Check it running at night, pop the hood and start looking for electric arcs.

[Stringer]

I have. It don't take much to mystify me, but I'm stumped.

[hayhauler71]

If you can feel the dist cap when running and it feels like its knocking you may have a bad bushing in the dist and the rotor may be making contact with the cap then you will need a new dist

[Stringer]

I found it. I was looking at electrical in the area of the distributor, but I was off by a foot or so.
Pardon my laziness - and it's too dark and too late for me to research it more tonight. What's the relay on the right in the pic...closest to the firewall? AC related, I believe. I realize my truck's had parts of a newer model driven under it, but I think this is "a '72 part". Still don't know why it would pop when hot. Thanks again.

[beautimus]

No sir, that is not a 67-72 part. it must be related to the engine and trans that you have. Thats all i can offer on this. Maybe someone can chime in with more. Best of luck to you. Tony