01 Ram backfiring???

I have a 01 Ram 1500 4x4 w/92,000mi. Wile the motor is warming up I have a bad backfire through the intake,If I floor it it goes away but soon comes back, but when she warms up the backfire goes away. I just did a tune up about 20,000 miles ago. Thought I had a bad spark plug, changed them out but still have the prob... I'm stumped ant of ya'll have an ideawhats going on or have you heard of this before?
Thanks in advance.

you may have a sticky intake valve.