Backfiring thru exhaust

[KEH]

My Explorer backfires thru the exhaust after running for a short period of time. When first starting cold it will run for 20-30 seconds and then a single pop in the exhaust...A few seconds later another pop...And then it starts to sound like a machine firing.

Any ideas what the problem could be?

Thanks

 

[KEH]

I replaced the coil pack, but it did not cure the backfire.

Any thoughts on this?

Thanks