It has a Power Commander under the seat but I don't know how to adjust it so have just left it alone.
Power Commander for Dummies. https://itstillruns.com/adjust-power...r-7512715.html
From the Power Commander site:
Deceleration popping in the exhaust
Decel popping is caused by the detonation of unburned fuel in the exhaust pipe. This happens with high flow exhausts that allow more fresh air to be pulled into the pipe, causing the exhaust temperature to rise and detonate any unburnt fuel. Ways to reduce and possibly eliminate this popping are listed below and can vary between Harley-Davidson motorcycles and others. Exhaust leaks can cause decel pop, make sure the exhaust is sealed completely at the motor, and at any other connections.
Harley-Davidson: 1. For older Magnetti Marelli fuel injected bikes, adjusting the fuel table to remove fuel can help reduce popping. Highlight the zero percent column from 2,000 rpm to redline and enter in values from -40 to -50. Run the bike and see if the popping has reduced. Sometimes, the throttle position may not show zero at idle and should be correctly adjusted with the bike at full operating temperature to make sure it is reading zero.
2. On newer Delphi fuel injected bikes, we have found that adding fuel is needed to reduce popping. Highlight the zero percent column and instead of using negative numbers, try adding fuel to the map. Start with putting a value of 25 in the cells and see if the popping has reduced.
3. Adjusting the ignition table could also help in deceleration popping. Advance the timing in the zero percent column 3 to 5 degrees to see if popping has been reduced.
Metric and other: 1. Block off the fresh air injection systems. This will reduce the air being fed into the exhaust, causing the higher exhaust temperatures and detonation.
2. Adjustment to the fuel table in the zero percent fuel column from 2,000 rpm to redline, may also be needed to help reduce popping. A value of positive or negative 15-20 should be added and can reduce popping if it is still noticeable. Further adjustment may be needed, try increments of 10.
Do not do any changes below 2,000 rpm due to the fact that it could affect fueling at idle."
Power Commander Motorcycle Fuel Injection Tuning Module
This is "popping in the exhaust" rather than the airbox.