Odd noise from rear of truck

I went out into my garage to grab something and heard a humming noise. Sounded like a small compressor or something and I thought maybe it was outside, so I went outside and did not hear it... had to be coming from in the garage (door was closed). So I go back into the garage and walk around, finally crawling under my truck where it got louder.

My truck was off and cold. I hadn't driven it for over 8 hours. The keys were not in it, and it wasn't on. Something was humming like a small pump or compressor or something. Then it abruptly stopped.

It was from the middle or rear part of the truck... what the hell is back there that would make any sound like that, or cycle on and off even when the truck is off and cold?

 

Fuel pump keeping lines primed.

 

X2

Welcome to the future. Where everything is controlled by software and nothing really ever turns "off".

 

Thanks guys. Had never heard it before, so I was like... WTF is that sound!?

 

Your evap. emissions system is running a check. The ECU is commanding a a vacuum pump to run an look for leaks. Its usually hours after the truck is turned off. Wait until you are working on the truck sometime when that thin starts to run and do its thing.

 

Yup, it's your emissions system doing a self check. Some systems require certain parameters to be met before the ECU initializes a systems check. For instance, if you never take your vehicle up to freeway speeds, you'll never pass SMOG, because the vehicle never met the parameters for the self check.

 

Now that is interesting!! I was not aware of this. As I'm driving less as I get older I'll try to get this on the highway occasionally and especially before I get an emissions check.