Intermittent Vibrations

Does it change with the type of app you are using? Example: Watching videos or doing video-conferencing works the CPU and GPU hard and that creates higher temps. The automatic thermostat function will tell the fans to spin faster to move more air. An app with more modest hardware demands, say checking email, may not cause as much heat and fans will be running more slowly, there fore less chance of notiecan vibrations.

The only moving parts inside are the fans. You have two. It's possible that they are not speed-matched and you are getting a harmonic vibration. That would be intermittent and short-lived like you describe. It could be as simple as one fan with dirtier blades than the other.

I would try running Apple Diagnostics:

Use Apple Diagnostics to test your Mac

If something serious is at play, it will report a code. AD is rather good at picking up fan issues, including temp sensors that .

Apple Diagnostics reference codes - Apple Support

First, let’s see if this issue happens in Safe Mode:

1. Shut down your Mac.
2. Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.
3. Select your startup disk. It's named Macintosh HD, unless you changed its name.
4. Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.
5. Log in to your Mac. You might be asked to log in again.

If the sporadic vibrations do not occur, then it may possibly be a software issue.

If the issue doesn't continue in safe mode:

Leave safe mode by restarting your Mac normally, then try to reproduce the issue.

If the issue doesn't return, it might have been resolved by the other things that safe mode did at startup, such as checking your disk and clearing caches.

Glad my suggestions have helped you!

👋🏼😉