What causes this sound....

What causes the sound...I have a 90 mustang gt 302...sometimes but mostly when I first start it up in the morning I get this poping sound that sounds like setting off a fire cracker inside of a can, it's kinda like a deep hollow sound. Someone had told me it might be a hole in the exhaust ? I'm not sure though, anyone have any ideas what this might be ?

Thanks

It starts right up, has a rough idle for a few seconds but when I let off the gas and acts like it wants to stall I'll hear that poping sound like someone put a maxwell can under my car and set off a firecracker( if you can imagine what that sounds like) but that's the sound I'm getting.

No backfires, don't run cats, if the car is just idleing it doesn't do it, only seems to do it when I first start it up giving it some gas.

I took my car to the local AutoZone and was told I couldn't pull the codes since my Mustang is a 90, they can only do cars 96 and up

A friend of mine seems to think I have a blown head gasket since I'm draining coolant very fast...no fuilds in the block, no leaks on the ground...only fluids hitting the ground is through the exhaust pipes.

I'm getting ready to pull the heads and I'm hoping they are still in good shape, no cracks or warpage.

Thanks