You could try TThtottle. This controls BOINC based on your desired CPU temperature and throttles it as much as needed to stay below.

Is your fan loose and vibrating? Ultra low quality?
It shouldn't be loud enough to annoy you.
If you can replace or repair your cooling you should be able to contribute with 100% on all cores.

All computation creates heat. The newer the processor, the more it can do with the same heat output.

If you tell a computer to run at full speed 25% of the time, it will create a quarter of the heat, but other than passive cooling, you're gonna need fans.

Have you tried running CPUID HWMonitor? I've successfully run it on OEM Dell machines, FWIW.

The issue you're running into is that the Dell installed cooling solution is likely not able to keep your CPU cool at anything close to a full load (i.e. all cores at 100%). A DELL Inspiron deskstop isn't really built to run at high duty cycles 24/7 . . . you'd need to go with a DELL Precision or HP Z8XX type OEM desktop for high duty cycle use.

It sounds like at a 25% duty cycle, the DELL installed fan curve is pretty aggressive and goes to 100%. Until you see the CPU temps, you have no way of knowing if fans at 100% is necessary when CPU load is 25%; hopefully HWMonitor will work so you can at least see temps.

Another issue with a DELL motherboard is that it's very unlikely you'll be able to change the fan curve, i.e. is sounds like you've hit a dead end here, too.

Assuming the DELL OEM motherboard has a standard fan connector for the CPU heatsink/fan, you could try replacing the OEM CPU cooler with a Noctua or similar CPU cooler (you'll also need thermal paste like Arctic Silver or similar).

I experienced the same fan speed problems with a HP Pavillion running a Athlon II x4 630 processor. CPU fan would spin full speed at 100% usage. These machines aren't designed for crunching. Sold it locally. Surprising my electric bill went down a bit.

I do have a Inspiron 660 with a Intel i5-3330 running Ubuntu that is whisper quiet at full speed. It cuts through ZIKA, SCC and MIPS work like a hot knife through butter. You could probably get one of these cheap on the used market.

My guess is your Inspiron 570 has a AMD Athlon II x4 CPU in it also. If so, it's time to retire it. Newer technology does more work with lower power usage. I recently sold off two AMD Athlon II x4 machines. I'm getting ready to retire a couple Intel Pentium/Core 2 Duo machines when support for Win 7 ends in January. My plan is to add a AMD Ryzen 3 machine later this year.