I just bought and installed a Bosch 500 series single wall oven a couple of months ago. There was minor shipping damage where the top of the oven was dented in a little so I took the sheetmetal off the top of the oven and bent it back into shape. I also noticed a little centrifugal fan on top under the sheetmetal. I'm not sure, but I assume that's the electronics fan. If I had left the sheetmetal dented in and rubbing against that fan, I'm sure it would have been very loud. At a rough guess, the electronics fan is slightly louder than my 49dB DW, though I haven't really compared them back to back. I can tell when the fan is on, but it's not that loud. It's certainly no match for my toddlers. Also, since it's a fan, it's mostly just white noise (vs. the rhythmic noise of a DW) It is a bit disconcerting how long the electronics fan stays on. I think it's actually based on the oven internal temp, since it did turn off sooner once when I had the door open trying to clean up a spill. Finally, one other thing I noticed was that two of the sheetmetal screws were partially stripped out- one of them being one of the two fan retaining screws. If the fan was loose, that certainly could contribute to increased noise and rattling. I did go out and buy slightly larger sheetmetal screws, but didn't get around to putting them in. It is nice having the peace of mind that I could screw some bits in tighter if there was any rattling, though pulling the oven out would be a bit of a pain.

Neil: Also in NorCal here and after a year of keeping records, I can safely say our attic fan does nothing at all for keeping the place cool, and it's a good one. We put in extra venting and the temp in the attic is considerably lower now. We have a very well insulated space now (post-remodel), with a very good radiant barrier, and have only run the a/c once so far this year, for about an hour on the third day in a row above 100. I've kept track of temperatures in the attic compared to the outside and inside, and on days with similar outside temps I see essentially no difference in the inside temperature whether or not the attic fan is on. Attic temps were 3-5 degrees lower with the fan on, but that doesn't seem to make much difference for the inside temp. I've read that unless you have a very tight house, the attic fan can suck cool, conditioned air out of you living space, as well as out of the attic, meaning the fan actually makes things worse. I'd advise putting in the extra venting first and then see if you still want to try a fan.

Sound in a building is transmitted in three ways: Airborne transmission - a noise source in outside or in one room sends air pressure waves which induce vibration to one side of a wall or element of structure. One sound control method is adding mass into the structure, such as a heavy dividing wall, which will usually reduce airborne sound transmission better than a light one. Another common solution in light framed construction is mounting wall and ceiling boards on resilient clips, decoupled from framing members. Air tightness also becomes an important control technique. Sealing all penetrations on walls where airborne transmissions occur is important. Impact transmission - a noise source outside or in one room results from an impact of an object onto a separating surface, such as heat pump, air conditioner, furnace, fan unit, washer or dryer, etc. Acoustic control measures usually include attempts to isolate the source of the impact, or cushioning it. Duct isolators often help reduce sound transmitted via ducting. Flanking transmission - a more complex form of noise transmission, where the resultant vibrations from a noise source are transmitted to other rooms of the building usually by elements of structure within the building. This is more common in steel and concrete framed buildings than in wood-framed ones. Thus, one needs a combined noise reduction strategy if the sound is a significant factor. Good luck on your project.

Sounds like a fan blade scraping against something metallic.