I had a buzzing problem with my amiga 500, and it turned out to be the the outlet I had my amiga plugged into. Once I changed outlets the buzzing went away. Try changing outlets. Also, make sure all the chips and components are properly seated.
Yeah, I had that buzzing sound on Amiga 1000 but it only happens sometimes. Seem like some grounding issue. When properly grounded, there's no buzz. That third wire on the 3-prong plug was the culprit in my case. If I used that undefined or vaguely defined third wire on the AC plug, I sometimes got that buzz sound. I don't feel like ripping up my walls in my house to find out where that third wire is going.
Capacitors? There's some myths to that as well. I have read that electrolytic capacitors last only 3 years, but my Amiga 600 has been running for more than 15 years. Someone needs to rewrite some of the crap flying around on the internet or make it more precise.