Start here — before replacing any parts
First distinguish trapped air from a damaged impeller. Gurgling that clears after priming is not the same as a persistent hard rattle.
This information comes from a shared HW-series reference; check the UV module, rotor and electrical rating fitted to your HW-304B.
Manual scope: Confirm impeller, air ingress; no speculative parts
Do these checks in order
Follow the steps below and keep a record of what changes after each one. Stop when an actual cause is confirmed.
Unplug the unit and inspect both hose connections for looseness, damaged seals or air being drawn in.
Use the model’s priming procedure until the canister is filled and the intended circulation path is stable. Do not crack open a powered water filter.
If the sound continues with adequate water circulation, unplug and inspect the impeller magnet, ceramic/metal shaft, bushings and impeller cavity for chips, grit or distortion.
Inspect the lid gasket/O-ring, then restore its seating and lubrication only as the manual approves before closing the unit.
If the problem is gone
Noise disappears after proper priming: retained air or a leaking connection was likely the cause.
If the problem is still there
Noise stays in a filled, unobstructed unit and parts show damage: replace only the confirmed rotor/shaft/bushing component for your HW-304B assembly.
Which cause matches your result?
Use these more specific checks only when the earlier steps point to that possibility.
01Air is entering at a hose connection
Check whether a hose fitting is loose or drawing air.
Reconnect the fitting and tubing securely.
02Air is trapped inside the canister
Listen for startup gurgling and follow the manual's priming instructions.
Vent trapped air using the specified procedure; do not open a running canister.
03A seal needs service
Unplug the filter and inspect the O-ring condition.
Lubricate only as approved for the seal or replace the damaged seal.
04The impeller, rotor or shaft is damaged
Inspect the removed components for chips, cracks or unusual wear.
Replace only the component matched to your specific HW-304B revision.
How can I tell it is actually fixed?
After reassembly and complete priming, listen for the same noise and verify normal water flow and no leaks.
Before buying a replacement part
The HW-series has different impeller and gasket assemblies; confirm fitted part numbers.
Where this guidance comes from
Primary reference: View the manufacturer or manual reference ↗
Applicability: Confirm impeller, air ingress; no speculative parts. Source evidence: Matching HW-304B owner question plus manual.
The instructions are original English troubleshooting explanations derived from the listed technical facts; they are not a verbatim service manual. Do not apply them to an unverified revision.