Noise when idle, foot on brake and truck in Drive

This is my first post. I'm new to this site. I'm the original and only owner/driver of a mint condition 1999 Toyota Tacoma PreRunner. It's my baby and I treat it like it's still brand new. Anyway, I've started noticing an unusual noise when the truck is idle, still in drive and I have my foot on brake. If I put it in park, the noise stops. I would describe the noise as somewhat of a continuous low-pitch hum. Does anyone have any ideas as to what this might be? Thank you.

 

Might be a motor mount going bad. There are 3.

 

might just be the convertor shield humming

have someone put in gear (or put parking break on) then go around truck to find the source of the noise, it might be as simple as an exhaust leak from a cracked manifold which is almost guaranteed to happen on all of these 4 cylinder trucks

 

Thank you