Deceleration noise?

First id like to introduce myself, my name is Stewart, i live in OKC, and i drive a 67 mustang, 200i, 3 speed coupe that my grandfather bought new and has put 265,000 miles on it in the last 43 years. The car is completely stock, rust and all!

Which brings me to my first question/problem, When I'm driving in 2nd or 3rd and take my foot off the gas but leave the clutch engaged there are these kinda pop sounds almost like backfiring but defiantly not loud enough. This only happens when the clutch is engaged and if i disengage the clutch the sound seems to stop. The car did not do this when i first got it. Since then Ive removed/cleaned and reinstalled the following. Valve cover, oil pan, T housing, Radiator, fan and spacer as well as the fan pulley, distributor cap, battery and voltage regulator, changed oil and filter and flushed the coolant system. I haven't touched the carb but i did use a timing light in an attempt to rectify this deceleration problem. Turned the dis to 6* below tdc which i believe is stock ignition timing

Please any help or insight would be greatly appreciated.

It is the sound of fuel being pulled through the engine into the exhaust, and then exploding in the head pipe. Happens more often when cold on a vehicle with AIR pump.

Can also happen with a small exhaust leak. On deceleration it will be rich and if it pulls in fresh air from a leak, it will light off inside the pipe and pop. On a racecar with open headers, it will pop a flame out of the collectors.

could be the exhaust ring or doughnut, kinda what i was thinking but i don't understand what that has to do with the clutch being engaged or disengaged? But i will def inspect the exhaust.
Thanks for the insight

8) when the clutch in engaged you still have a load on the engine, and since you have a load, the engine is running rich, and you are getting after burn. advance the ignition timing until you get the best vacuum reading, and just before you get pinging. that will eliminate the after burn. i had the same issue with my 66 falcon.

you might also lean out your idle mixture as well.

Thanks for the help, I advanced the timing and that helped with the problem. Ill try to adjust the curb idle today. A half turn at a time right?

8) would start by turning the mixture screw in until it just touches its seat, then back it out 1 1/2 turns, and adjust for the smoothest idle from there.

Thanks for the help!