After locking my car I could hear a buzzing sound comming from under the bonnet on the off side. After some investigation, I desided to disconnect the battery, the buzzing sound continued. Not knowing what the cause could be I reconnected the battery. The following morning there was no noise, so I drove the car to work, still no noise. After arriving home from work, still no noise, I thought that maybe the noise had gone. Latter that evening the buzzing noise returned. The buzzing tends to stop for short intervals. My car is a Volvo S80 D5.

Under the left side of your back seat is a fuse / relay box . When you first turn the key on the fuel pump relay turns on for a few seconds to prime the fuel system . That relay is in that fuse / relay box ,that is what your hearing . Does the vehicle start an run good , no problems other then the buzzing sound .The vehicle battery is also under the rear seat on the right side . How To Locate and disconnect Battery Buick Lesabre Pontiac Bonneville 00...

The buzzing will be on the abs pump under the bonnet

Sound is completely normal on all Audi's.
Your A8 uses an Orifice tube to regulate the pressure between the two halves of your air conditioning system. A hissing sound is normal after a shut down as the system equalises. Usually lasts about a minute and is more noticeable on warmer days.
Location on an A8 is on the very right hand side of the bonnet just under the windscreen - that's where the hissing will be coming from.

If the buzzing is sound is coming from the doors, then the motors are going bad. If the buzzing sounds like it's from under the hood, then check the relay switch and see if it went bad and needs to be replaced.

Not sure if the solution can be applied to 2006 focus, I have an European Mk2 Focus which also having the same problem. Here is what you can try.

To Open:
1. Jack up the car on the lifter.
2. Open bottom engine cover.
3. Loosen two (10mm) screws which secure the bonnet lock assembly. The screws located between air manifold and radiator.
4. Once the screws released, disconnect the alarm cable/socket connected to the lock assembly.
5. Try pull open the bonnet, it may require little force to pull the bonnet open due to a little white plastic link (~1" long, plus (+)shaped) is still connected between the lock assy. and cable assy. Failure to release bonnet lock is only caused by this plastic link!
6. Once the bonnet opened, the cable assy can be detached by pulling it from engine side out. Inspect the plastic link if it worn out.

To Install:
1. Attach lock and cable assy. Ensure the plastic link is properly inserted in its place between lock and cable assy. Test the mechanism with the car key.
2. Insert the whole assembly from engine side out.
3. Secure with two 10mm screws, reconnect alarm cable.
4. Reinstall bottom engine cover.

Hope this help. Sorry for my English.

further investigation is required, take of side panel to see what has become detached.

If it's buzzing when the car is moving. it could be the wheel bearing making the buzzing sound. it needs to be replaced. you can probably hear it as being inside the hood.

My volvo v70 has three places for fuses and relays, one under the bonnet, one on the end of the dash board on the drivers side, and one the boot on the left hand side you may find what your looking for in one of these places

Sounds like a motor, or a s stuck mechanism. Either way the door panel must come off and this is no easy task.