Why is my cpu fan soo loud?

Probably.
It would help to know a lot more detail though. We can't be guessing about what hardware you have.

Get a copy of HWMonitor and let us know what your CPU temps are and how fast that fan is actually running.

Ok hardware is:

4GB DDR2 G.Skill Ram
Antec earthwatts 430w psu
Seagate 400gig HD
Radeon HD5670
Pentium D 945 3.4 cpu
Intel i945P/PL/G/GZ mobo

And I'll install HWMonitor and let you know what temps, rpm, etc my cpu is at.

What case and how many fans?

It's not usual to have a just new power supply affect the speed of the CPU cooler fan.

I wonder if the CPU cooler might have got knocked around, jarred when the case moved or otherwise shook loose so it's not contacting the CPU correctly. If the CPU is running hotter than before - it's what is making the CPU run faster trying to cool off that hot CPU.

Case is Rosewill Blackbone. And I have one front LED 120m fan and 1 80mm rear fan and of course the cpu fan. When I had my old PSU in it, it was a lottt quieter.

I'm wondering if you tested with the old PSU now if the fan would run quiet again.

Pentium D 945 3.4Ghz will be a very hot chip and need a lot of cooling.

perhaps it is the psu making the noise, a 430W psu looks only a little light but it might be forcing it to spin its fan up to keep itself cool.

My cpu fan is running at 3300 rpm, 40C!! Isn't that high?? What do I do?

It's not the psu fans, it's the cpu fan for sure becuase when I put my ear up to it I can clearly tell it's the cpu fan. The fan on the psu is VERY quiet

Should I try to hook up my 400 watt psu back up? I hope it's enough wattage because I added a new video card.

It's a high idle temp - that's for sure. And its probably the reason your CPU fan is screaming along at 3300rpm instead of the more normal 1000rpm.

As long as you're just testing with the video card in idle load it should be perfectly safe.
It's only when you'd crank up a 3D gaming session that the 5670 would draw a lot of power.
There's a good chance your 5670 draws less power (about 15 watts) at idle load compared to your old card.
I think you'll find that the CPU fan works the same speed no matter what PSU is hooked up.

Check that in changing out parts the power cable to the fan on your cpu didn't get shifted around. What happens a lot of the time that causes this noise is that the blades are actually hitting a wire. If that is the case just pull back the cables and tie them in a way that they aren't going to be close to the blades.

None of the wire or cables are even anywhere close to the fan. I don't know what the problem is.

check the bios to see if there are any options for controlling temp. I'm thinking that during the full power outage caused by the psu removal, the bios settings might not have been maintained, for everything else cpu, memory, hdd, they would auto configure.

it would be an indication that your cmos battery is dead/dying? which given the age of a d945 it could be.

You should e able to control fans rpm on bios, under the "health" heading-
Try to lower them /

You don't want to arbitrarily lower your fan speed, especially with such a high idle temp. The CPU cooler coming loose as WR2 suggested might explain that.
And, an Earthwatts 430 wouldn't even break a sweat on that system, so no worries there.

My thinking is that the default would be max on, all the time, as opposed to temp related, but the OP needs to be looking at the settings.