So I just bought this accord and I noticed that there is a constant humming sound that I can only hear from inside the car. It's pretty loud and it sounds like it's coming from the engine. The hum matches rpm and also is able to be heard when in park. I've had the alternator tested and it was fine. The previous owner says the transmission has been replaced and has about 150,000 miles on it. It doesn't slip or anything. I'm at a loss. Please help.

 

Make yourself a "Redneck Stethoscope" (a short(ish) length of hose) and move it around in the engine bay until you find the noise. FYI, problems like this are virtually impossible to diagnose over the internet.

 

You've got that right. To some people a hum might sound like a hum, while others might hear it as a rattle or something different. And trying to describe it in words is even harder. It could be almost anything from a solid engine mount, to the electric engine mount found on AT cars.
You might try posting a video of it and posting it to you tube.

 

I posted a video of the sound on YouTube already.
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As stated a "Poorman's Stethoscope" will help narrow down where the noise is coming from....alt can test good but is bearing is the issue????....PS Pump, notice any foam/bubbles in the res, like it might be sucking air, ect.

 

​​​​​​​No bubbles at all. Just a smooth swirl. Also no squeaking from the engine. Like I said I can only hear the sound inside the car and it sounds like it's coming from underneath the passenger floor board.

 

How about an exhaust heatshield bent around & drumming against the underside of the floor??