My psychic abilities are clearly not working so a model year would be a useful detail as there are production changes in the cars. In particular late cars do drive by wire idle control. Whether or not it is OBDII is also a useful detail.
Did you replace the coils or just the gaskets (you did not specify). If you just replaced the gaskets it may be too late if water has caused corrosion on the coils. Pull the coils out and examine the metal base of the coil body for corrosion and look for signs of flash-over tracking on the very end of the long coil nose. If either is present, sometimes you can clean it up; but, safest to replace.
That said, coil failures result in misfires which can cause an erratic idle and can sometimes cause that pop you hear. However, misfires do not result in consistently low idle speed unless you are misfiring like crazy which will generate error codes on an OBDII car. Note that a slightly erratic exhaust note seems to be a normal feature of the NSX - at least on my 2000 C32 even with new spark plugs and pristine coils.
I also noticed sometimes when going into neutral it will sometimes go to very low rpm like car is going to shut off.
That might be a problem with the ECU failing to go into idle control mode. The ECU gets inputs from the brake switch, clutch switch and transmission telling it when to implement idle speed control. If those inputs fail the ECU does not enter idle control and idle speed may be out of range or the engine may die. Come to a quick stop at a stop sign and having the engine speed dip way down or die after you release the clutch is a common sign of failed idle control. If you find the service manual for your car you can find details on checking idle control.
Remedies will depend on whether your car is cable or drive by wire. Cable throttle bodies have the EACV idle control and the little thermal warm up valve and if dirt / crud gets into the passages that can cause idle control problems.
So much of where to start and things to check depends on the model year of your car