OK, I'm back.

Based on your responses, I pulled the hardwood floor on teh second floor (popping & crackling). It's very labor intensive, btw. I don't recommend it.

Now, I've installed about 50 square feet in one room. I used 15# felt paper, and I adhered to the proper fastening schedule. Evey 8-10 inches, and 2-3 inches from each end. The result: There is still some crackling and popping sounds. Mostly just popping sounds. Not on every step, but perhaps one popping sound every three steps.

I did the same thing on the first floor, and there is no crackling and poppign sounds on the first floor. The only differences are:
1. I added insulation in between the first and second floor (R30, which is about 8.5" thick) I think there is still about an inch or two between the top of the insulation and the plywood.
2. The initial wood installation on the second floor was made with very low temperatures 40-50's. But I started the installation, with a temperature of about 67 degrees. And its been warm the past couple of weeks or so.

Also, the floors are not perfectly evel, but they are about the same as the first floor, and the first floor is not crackling or popping.

I did notice a few random popping sounds on the first floor (over a 700 square feet area) and these sounds went away with time. Could the same apply to the second floor ? If so, how long should I wait to see if the popping /crackling sounds go away ?

I'm hesitant to continue, if the popping/crackling sounds don't go away.

Any advice is greatly appreciated !

Thanks,