Weird noise

[wm69]

Just got my wife our first 4runner about a month ago. It's an 01 LTD 4x4. Got 62K on it and was a Toyota certified used vehicle. It's in great shape and everything I expected it would be (been driving Honda's for years, but they don't make an SUV I like). Question is this, the 4runner makes a weird whirring sound, followed by a pop. The sound is intermittant, and it almost sounds like some kind a acutator or something. Seems to happen most often when brakes are applied, but it only happens once after the car is started and then it doesn't happen again until the car is shut off and restarted. It sometimes happens soon after the car is started, and sometimes 4-5 miles down the road. The things got a warranty but powertrain only and it's got a deductible. For all I know it might be a normal noise of something activating that's just gotten louder than it's supposed to be. (my honda ABS system was prone to make funny noises sometimes, after 10 years I just pulled the fuse and had manual brakes). Anyhow, any ideas on what the noise could be coming from?

 

[cracker]

This noise has been the source of much discussion recently.
I have the same vintage and the same noise.

It is the hydraulic brake booster pump. It was introduced in 2001 on the 4runner and in 2000 in the Land Cruiser. It replaces the vacuum boost system used before. When the hydaulic brake fluid pressure drops below one threshold, the pump turns on until the pressure is above another threshold, then it turns off when a relay closes which is the loud thwhack you hear.

It's still there in the 4th gen T4R but quieter. The other thing you might notice is a slight vibration in the brake pedal when the pump operates - also normal.

 

[wm69]

Thanks! I figured it was something like that. I just got through putting relays on the KC's on my two jeep CJ's. I wondered if it might be a relay that was malfunctioning, but I guess it's just a loud relay. You really can't hear it with the radio turned up, but I'm the paranoid buyer who will drive with the tunes off so I can listen for all the possible problems there could be with the car. Thanks again for the answer!