I am driving 2012 Mazda 3 GS SKY (Manual Transmission / ODO:200000km).

I can feel vibration from seat when the car is at idle.

RPM doesnt seem to move when it shakes at all though. Very weird.

The RPM at idle is 650 rpm.

I opened the hood and can feel some vibration when I touch the engine by hand.

3 days ago, I visited Budds Mazda (Oakville) and ask the technician to do intake system cleaning.
They used chemical (spray-can like seafoam) to clean the intake system.

However, the vibration is same or a little bit bigger after the intake system cleaning.

I changed spark plug (NGK laser iridium) when the car hit 130000km.
But I did not change the ignition coil.

Do you know the cause of this problem.

Please give me some advise for this problem .