My monitor is making suddenly a strange sizzling noise. I restarted it, but the sound comes back...

Please contact AppleCare and they will let you know where the closest Apple Authorized Service Provider (AASP) is in your area. Use the following link to find the phone number for AppleCare in your area. AppleCare contact Information.

BTW, if the computer is a 2013 or earlier it is considered vintage and parts will be difficult or impossible to find. If your computer fits that time frame then I would recommend spending your time and money replacing the computer.

You need to call AppleCare ASAP. AppleCare contact Information. Don't worry it is covered by the 1 year warranty. Hopefully when you bought it you also bought AppleCare which extends the warranty to 3 years total.

Turn off the computer and make an appointment at your local Apple Store or AASP and take it in to determine if it is a noise you left on or if there is a hardware issue.