320d E90 2010

[qkhcad1]

Recently bought this from second owner. I’m the third owner. Now just hit 100k mileage. After driving for few days I noticed a deep humming sound from left front side of the engine when I’m in the car. This will happened only on first three gears at 1500 to 2000 rpm. I asked previous owner but he said never heard of it. He asked me to check when service due at 101k mileage. I asked Mr Google and found only one person posted at e90post and it was due to a plastic connector from the air box to the middle front of the engine. I haven’t open up the engine cover by external inspection myself did not see any obvious joints that is disconnected. Have any of the owner experience this? Thanks in advance for your time and comments.