Hi austink :wave:
Some of the fans are temperature-controlled (usually the ones on the motherboard), so the speeds will vary accordingly.
Firstly, check the warranty to see if opening the PC breaks the guarantee (Some have a paper label stuck on so opening the side-panel breaks the label)
If that's OK, check around the fans (cooling-fan on CPU, possibly on the GPU (depending on whether it has a graphics-card or 'onboard' graphics chip), rear of the case, possibly top of the case and possibly front of the case) for stray cables catching against the blades, they might have become dislodged during transit.
If that's clear, with the power off, try gently wobbling the fans to see if there's any movement in their mounting, they usually have a small screw in each corner of the frame to hold them tight, a screw might have come loose.
If they all seem tight, with the power on and making a noise, very gently rest a fingertip against the centre hub of each fan in turn, not hard enough to stop or seriously slow the fan, just enough to vary the sound they make - It could be a faulty fan with loose bearings.
Post back with results :wink: