flying skunk dog

Hello all and thanks in advance. I just purchased an 01 4Runner with about 100K miles and it is making an odd noise from under the hood. This happens maybe two or three times everytime I drive it (5 to 10 mile trips). I appologize for the description, the sounds are where they seem to be from the driver's seat.
It has been making a humming noise from the driver's side near the firewall (near the brake booster?) that stops with a click on the passenger side toward the front of the vehicle (near the air intake/filter?). The hum lasts for 5 to 10 seconds and then the click.
Also, does anyone else have a problem with their truck idling rougher (not bad, just detectable) when the headlights are on? Could this be an alternator problem or do I need to add more grounds? The battery 'eye' looks okay and it starts fine.

Thanks again

alotawatts

Sounds like the normal/okay ABS system noise. Kind of a 'whirrrrrrrrrr snap' sound right ? If you just bought the truck check the braking system if you have not already. If you are talking about in city driving the frequency of hearing your noise sounds okay to me also.
Not sure about your idle. Check / test the alternator first ?.

flying skunk dog

Thanks for the reply.
Thats the noise! Tonight I noticed that it is when I am braking or right after. Good to know that its normal. I'll have the dealership check the alternator when they do the timing belt. Come on IRS refund......

GSGALLANT

Yep. That sound is normal. The 2001 and 2002 4Runners have a hydraulic brake booster instead of a vacuum brake booster. There is a small pump/motor unit that pulls brake fluid from the reservoir, and pumps it into an accumulator to keep it under pressure.

This "pressurized" section of brake fluid is what helps you apply your brakes. Everytime you press the brake pedal, a small amount of this "pressurized" fluid moves into your master cylinder and brake lines. When you release the brakes, the excess fluid returns to the reservoir. After applying the brakes a few times (4 to 6), there is a pressure switch that detects low pressure in the accumulator, so the booster pump kicks on (The whirring sound). When the pressure is back to normal, the pressure switch shuts the pump off (I think the passenger side clicking is probably the relay snapping off).

skjos

I love the search function.

GSGALLANT thanks for the information, this noise has been bugging me for awhile and it is good to know it is normal. The service department at the dealership couldn't explain the noise, it's good the guys at yotatech can.

Sorry for resurrecting the thread, but it's better than starting a new one.

Thanks

andrix

very good to know... i started a thread and got pointed to this one. Thanks a ton guys