car shakes when put in gear at idle

[sonicbimmer19]

Hi guys,

When I start my car, so far only when it's like first start of the day, the car shakes (not talking about small shakes, this is something a bit more shakey) when I put the car in gear. (manual transmission).
But after I wait a bit (30 seconds to minute) in neutral and put in gear, it stops.
And it does not happen everyday, but more than occasionally like once a month.

I've googled this and people seem to suggest that it's not spark plugs misfiring but rather transmission mount is loose/not good.

Anyone know that's the problem? thanks!