System noise vs. motor noise

[pugap]

I have a single phase, 60Hz, 1/2HP 8 pole motor/fan application which is experiencing a very annoying noise and vibration. Orientation is shaft down. The motor has been replaced with another of the same design, so it's either system specific of specific to the design of the motor. We've taken sound power readings and the highest frequency seems to be the 7th harmonic. Axial vibrations show as multiples of line frequency, and the radial measurements show the major peaks at 228 & 252Hz.
Trying to debug, but at a loss. Looking for ideas.
It seems to be system sensitive, because as we change the load on the fan, the noise changes. Have also tried a different impeller (3 blade vs 4 blade) and 3 blade is much worse.

 

[gsimson]

For such small applications, the Vibration spectrum may not be of much use.
It appears that the the motor and the impeller is Ok and the noise may be coming from the casing or duct.
Kindly check
Looseness in the casing & duct
Check any leakages in the casing ( Including shaft entry) and the duct
Check suitability of casing , aerodynamic characteristiscs
If You can run the motor & impeller without casing you can judge better

 

[pugap]

Thanks for reply. I should clarify application. Motor is in a condenser type unit, so there is no duct work. The grill does seem to be very 'excitable', but it's not clear whether it's caused by the motor harmonics or the airflow from the impeller. Have tried swapping parts, and nothing tracks. I have tried running the motor/impeller outside the unit, but I can't simulate the loads and get the same results.