Macbook air 2017 fan comes on real loud.

If you dig down through the links in About fans and fan noise in your Apple product - Apple Support, you will eventually find this passage from Keep your Mac laptop within acceptable operating temperatures - Apple Support:

"If your Mac laptop gets warm even when it isn’t doing tasks that require intensive calculations, or if it has fans that run for a long time, use Activity Monitor to check CPU activity. Look for malfunctioning processes that can put a significant load on the CPU and check for unexpected fan use. If issues persist, close and open the lid on your laptop, or restart your computer. ..."

Only after exhausting those possibilities should you suspect something amiss with the SMC, a hardware fault... those sorts of possibilities. The cooling fins can become very obstructed with dust before the fan noise starts to become objectionable.

Run Apple Diagnostics, which will exercise the fan and test that Mac's thermal sensors: Use Apple Diagnostics to test your Mac - Apple Support

Post a screenshot of Activity Monitor if you wish. Use the Energy pane, or CPU sorted by %CPU to see which apps or processes are using the most energy or placing the greatest demands on your Mac's CPU, respectively. To show the most relevant information I suggest selecting "Active Processes" from Activity Monitor's View menu.

The back area only needs vents where the fan is located to exhaust the hot air.

It is possible the thermal compound on the heatsink is dried out which can increase system temps by about 10C, but I wouldn't expect the fan to kick on to high speed.

Usually when Macs have the fan running at high speed, it is either due to a faulty thermal sensor, or some third party software making the computer to work harder which heats it up. You may want to run EtreCheck and post the report here so we can examine it for clues.

You can also try running the Apple Diagnostics to see if any hardware issues are detected. The diagnostics are usually fairly good about detecting issues with the cooling system...so if there are no issues with the cooling system, then I would focus on software issues. Make sure the power adapter is connected when you run the diagnostics.

Also make sure to disconnect all external devices in case one of them is causing a problem.

You can also try an SMC Reset and a PRAM Reset (hold the PRAM Reset for at least three chimes....this is unlikely to do anything since the SMC Reset is the one which deals with power related issues).