My Alpine Air 880 makes a sizzling noise. I turn the dial down and it gets better. Whats the problem?

The sizzling is the high voltage "bleeding" to ground inside of the collector module, or the high voltage electronic circuit.

First, unplug the unit and remove the module. Thoroughly clean all electrical contact points where the module makes connection. If it possible to operate the unit without the module in place, then replace all covers, plug the unit in, and attempt operation. If you continue to hear sizzling, then unplug the unit, remove covers, and continue to clean, digging deeper into the unit to find traces of accumulated dust/dirt. Also be on the lookout for burned areas in the plastic, around insulators, etc. High voltage is a mighty beast, and will easily track to ground, causing a sizzling noise.
If you don't find anything there, then take the modele and look for any tracking points on the insulators. Look very closely at the ionizer wires for accumulated lint, which creates little points where high voltage can bleed to ground from.
You may get popping of high voltage as well. When this happens, note the places where it occurs, and those places become suspect of causing the sizzling sounds.
Let me know what you find. These guys are cantanckerous at best.