Weird failure: Loud Sweep Tone on SMPS, ideas?
The PBF suffix at the end of IRFBC40APBF means lead free/RoHS-compliant. It should be a one-to-one substitution with the original part.

I found a web site where the same symptom was described. It's evidently common, and is called the 'falling bomb' effect. There are most likely some bad electrolytic capacitors on the main board in the audio section that need replacing. I would just go ahead and replace whatever electrolytic capacitors you're safely able to.

I also found a website with a repair guide: Classic Mac Tech Docs, v2.0 It has power supply schematics, etc. and is a good general reference. (They recommend replacing C1 as a precaution - your mileage may vary)

Exercise extreme caution in the high voltage circuits feeding the CRT. There are lethal voltages present there!