Receiver Buzzing:
This started with the IP4S when data is being loaded, you can run any app (I pick Speedtest) to load data and place the phone to your ear to hear it.
Bottom Speaker Buzzing:
This happens when a game is running, I noticed it back last year when I started to play Infinity Blade 2 on the IP4S.
Go into a totally silent environment, run a game (I pick Angry Birds Seasons), make sure the game vol is dead zero, silent mode on and lift the IP5 speaker to your ear.
Do the same thing when you've existed and killed the everything on the multitask dock, the buzzing is gone.
Infinity Blade 2 does some serious "snap, crackle and pop" sounds when you fast forward the cut scenes.