Ventilator

Also make certain the air system is not blocked by surrounding items in and on your work areas.

The space between the air intakes and the desk surface must remain unobstructed. Place nothing in that area:

Likewise, do not obstruct the exhaust vent:

NOTE: After handling a similar question and doing some tests with my iMac 5K, I found the tipping the display fully forward causes the stand to be too close to th exhaust vent. I was able, using temperature monitoring software, see that this configuration WILL raise the internal temps enough for the fan to speed up.

Are you running any anti-virus or so-called "cleaning" apps?

you don't need Avast it's not necessary on Mac OS it has built in protection, only Thing that we do recommend is

Malwarebytes, the free version is just fine, i have pro but only because it was brought over from my pc i use to have and it expires this year so i will be using free after. but that may be the reason for your fan noise .

Try to vacuum out the exhaust vents. Are you a smoker?

I use the antivirus program Avast Premium Security.

That alone can run up temps. You own a Mac—treat it like a Mac.