[mungo544]

You say it is louder with the radio ON? Is the sound actually coming through the speakers at all?

 

[mungo544]

Just a guess, but sometimes the power amp for the radio stays powered up even with the radio switched off. See if you can hear the sound coming from the power amp under the seat.

 

[mungo544]

No idea. Just guessing, based on the noise being LOUDER with the radio on. Absent the radio symptom, and the noise clearing after the first couple of stops, I would look at the parking brake system. Do you use your parking brake regularly? If so, try leaving the parking brake off, (as long as you park on a relatively flat surface) and see if you get the same noise. Regardless, If it is getting worse, I suggest trying to get a recording of the noise before you bring it in for service. Good luck.

 

[mungo544]

Do you ever use auto hold? If not, try turning on auto hold and see if you get the same noise. If it goes away or changes it in any way that may narrow it down to something with the braking system. (stuck caliper, defective brake hose, etc.)

 

[mungo544]

If the caliper is not releasing completely on one side the brake pad could be dragging creating a groan or boing sound as it fully releases. Some of the Hyundai cars have a brake caliper that has sliding parts and a slight rust build up can cause the caliper to hang up and not release properly when coming off of braking as when you are starting to getting going again. If it happens only when turning left it could be a caliper on the left. If it is happening when turning right it could be a caliper on the right hand side of the car.
If it is none of these things maybe you have a gremlin or a mischievous spirit living in your car's machinery.

A defective brake hose can also cause a caliper to bind up.

 

[mungo544]

I can't imagine what could be making a "boing" sound either (whatever that means). I am just guessing based on what you describe as the symptoms of your problem (happens during braking, the radio volume thing, getting progressively worse, etc). Nobody knows what it is, but you seem pretty sure of what it isn't. Hopefully, the mechanics at Hyundai have better imaginations.