It’s normal for it to happen within the first few seconds on the Dell desktops - most people don’t notice because they don’t know it happens. If it stays like that for more then 5-10 seconds and it’s a no boot situation, it’s usually a RAM or power supply issue. The startup RPM is not as aggressive as the Core 2 generation was, but it still happens and you can tell if you know what to listen for during POST.
The cheap common fault I’ve seen when it boots is someone did a fan swap and it provides insufficient PWM data since the machine is somewhat specific about what data it expects. When it isn’t providing RPM data, the fans assume there’s an issue and run them at 100% speed all the time, along with throwing a BIOS boot warning. If you see a boot error, start with the fan in question with these machines. CPU is 4-pin and rear fan is 3-pin and it needs to provide RPM data - stick to the Delta fans Dell uses as those provide the correct data.
For non-booting machines, the first thing I do is inspect the power supply for an issue (connection or physical problem) and replace it if it’s not due to something like the 4 pin 12V ATX/P4 power connector (P4=first machines to require it). That solves it most of the time with non-booting systems. NOTE: You have a huge issue - 2 different power supplies based on the factory CPU spec! Lower spec machines (i3/i5) use the stupid nonstandard supply like Lenovo, but the high end (i7) machines use the ATX 24-pin. PRAY IT HAS THE NORMAL ATX PART OR YOU NEED TO USE THE DELL PART.
If the power supply looks fine/non-standard unit, start with the RAM - that’s the second common cause for the full speed fan issue with a no boot condition. If you have 2 modules, removing one will usually work unless both are bad. When you replace it, don’t mix the good OEM modules with new retail ones - replace it as a matched set. Dell boards aren’t picky, but for normal users it just creates avoidable problems.
If none of those fix it, your motherboard is bad.