Could it be muffler/exhaust leak?

Hi,

I bought a used 08 Legacy 2.5i AT a month ago, and have been enjoying every moment of it.

However about a couple of days ago I started noticing a very mild noise coming from the bottom, but I don't know if it's from the engine or muffler. It's definitely not from the back.

The noise is like a small motor running, almost like flying bee, so first I though it was just natural engine noise but it started getting louder over the course of 2 days. It's very quiet at idle, but I can only hear it when driving over 20-30km/h. On a highway when I let go of the gas pedal the noise gets slightly more noticeable, and I could almost feel a slight vibration on my feet.

Could it be muffler leak, bad gasket, or exhaust leak?

It's got 92000KM (yes, Canadian), and everything else seems fine and car runs very smoothly. Can muffler/exhaust start leaking at such mileage?

Oh by the way when I rev at idle, at around 3,000RPM I could hear a rattle. Could that be related?

Also, I have extended bumper to bumper warranty. Would it be covered by warranty? If not, how much repair cost am I looking at? (just want to make sure it's not thousands)

I should take it to the dealer or local garage which I'll do next week, but I'd like to hear some insights beforehand.

Many questions, sorry but newbie here.

Thanks in advance.