I get that too on my 1991 240 wagon. About every third start. Only the very first one in the morning. Car runs great, I`m not going to worry about it.

That pop sounds like a backfire to me. I'd check and replace the distributor cap first. If there is fire jumping between any of the plug wire terminals while you are cranking, it can fire the wrong cylinder. If that particular cylinder has the intake valve open at the time, it can backfire (or "pop") into the intake manifold. This is not the only thing that can cause a backfire, but it is a likely cause. A lot of times, especially on the older models, this "pop" will blow loose a rubber hose on the intake system, and cause a rough idle, if it will run at all.
steve.

I vote leaking intake manifold gasket.

Starting a fuel injected car with the gas pedal pressed partway down can also cause a pop on startup. My wife drove carbureted cars before we bought our '91 brick, and she never got out of the habit of pumping the gas as she hit the key. The car would pop as it started.