Answer to: hard drive maximum sizeFor this machine you can find a 120gb 5500rpm drive or a 100gb 7200rpm drive faster one is by Hitachi, 5500rpm drives by Seagate and some others. IDE drives are hard to find anymore and I don't think you'll find a larger internal drive that will work in a G4 Powerbook. You can use a larger SATA drive as an external second drive in a FireWire external enclosure by MacAlley and have a boot able backup, too.
Answer to: Speakers making weird sound after HD replacement.Had same situation. It is easy to accidently trap a sound system wire under the top left hard drive bracket screw and not notice it. When tightened down and clamped hard, wire will pick up interference from the hard drive. Take the top left HD bracket screws out, free the trapped sound system wire. Re-tighten the HD bracket and your sound interference problem should be cured.