Clicking Noise Underneath Car Near Manual transmission
 

I recently bought a 2008 Mazda 3 Manual Transmission w/160k miles. Body is in great shape and engine runs fine. After a few days, I noticed a clicking sound followed by a loud humming sound thAt gets worse relative to the speed I'm traveling. Fast forward, it got very loud and scary especially on the highway, but I couldn't tell where the sound was coming from. My CV joint(axle) broke on the highway one day and got towed to the shop. My mechanic said the transmission seat (mount) broke, causing it to land on the axle and break it. We managed to get a used one from the junk yard and also replaced the CV joint. Didn't hear the loud noise again after and transmission actually shifted better. Few days later, the noise starts developing again and getting louder !! We just don't understand whats going on.

FYI: Before the mount broke, I realized anytime I cold start in 1st gear, the engine shuts off right away. I'd have tp start it again or in neutral. But doesn't shut off if I start in Neutral for the first time.