Popping noise from sub?Could it be the amp?

[tcsounds3hp18]

I have a 3hp PSI 18" in a 6 cubic ft box and I am running off of a rockford 200a4 and I am running it off of 2 channels bridged at 4 ohms. The popping just started happening today and thats why I am wondering what it could be I mean it cannot be over-excursion because its only on 100 watts rms at most and the sub is rated for 1.5k. And when I push on the sub with everything off I hear no rub at all and I feel no coil rub. I don't have another amp ATM to try it out but I'm gonna try it on my home theater amp tomorrow. One thing I will say is that my rockford amp is from 98 and its an A/B amplifier and it runs extremely hot so hot I could scramble eggs on the heatsink case I thought maybe it heating up and it being 94 degrees today that could have done it and messed up an internal component on the amplifier. Also the popping only happens on some notes and only at higher volumes so I don't know what it could be.