2006 Hyundai Sonata Engine buzzes on cold start

Could you desrcibe it as a fast ticking sound? Is the oil level full and has it been changed recently? Could possibly be a little slow to oil the lifters, this will cause what you describe time wise, but it is more of a fast click. If the oil is fairly dirty or due for achange, be sure to use the fatory recommended weight of oil and the correct high quality oil filter

Yes, it is a fast buzzing sound. I've only had the car a few days, so I haven't changed the oil yet (but the oil looks new on the dipstick). I'll give that a try though.

But it doesn't really sound like lifters to me, for two reasons:

1. While the buzzing seems to be coming from the engine compartment, it doesn't appear to be coming from the engine itself. The buzzing has a clattering characteristic that makes it seem to be coming from some other component.

2. Also, the buzzing ceases very suddenly (it doesn't taper off). In my (limited) experience, lifter noise tapers off as the lifters pressurize.

However, since the noise only occurs when the engine has been shutoff for several hours, it does seem to be related to a system that slowly depressurizes (which is why I suspected a fuel pump). But that's probably in the tank (in the back), I'm guessing.

Yes, the pump is in the tank. Check to see if the vehicle has air injection on it. If it does there should be a big check valve on the pipe that goes to the exhaust.I have heard the check valves rattle before, and the air injection usually only comes on when it is a cold start. Thisis definitly one thta you will have to listen around for to get a general area of where the noise is coming from