There is one thing obvious that amplitude on 0.25X is varying. can you take the high resolution data and see if there is any beat? have you feel any humming noise on site? Coinciding with process/nearby equipment? If you can share other spectrums, would be great.
Is it a real 0.25X line or just the sampling of a low-frequency ski-slope signature? Which type of accelerometer do you use, and integrator; because the noise in low frequency very much depends on the front-end.
In blowers, flow turbulence often occurs due to variations in pressure or velocity of the air passing through the fan or connected line-work. This flow disruption causes turbulence, which will generate random, low-frequency vibrations, typically in the range of 20–2000 cpm.
Blower fan vibration. Let's say your rotor RPMs are 1800 and the fan blade outer circle is twenty feet. The outer tips of your fan blades will be supersonic. This is a sustained sonic boom attached to the end of every fan blade that is striking the entrance to your duct work. This can create sympathetic vibrations that show a spike energy signature on your graph.
Have you inspected the following: Structural res., Foundation, Coupling (misaligned or loose), Gearbox, Housing open?
You don't put where it appears, if only in the motor or also in the fan bearings, or if it is greater in one or another part of the equipment. I understand that this is a horizontally arranged equipment. With the data you provide and being the type of machine you mention, I think you can have several possible origins...