I have an Eheim 2222 and I keep hearing an intermittent knocking sound. The sound does not sound like it originates from the filter but from the tank. I placed my ear on the side of the tank and the knock is extremely loud. Could it be the impeller? How far are the blades suppose to turn on the actual impeller itself? A little or a half turn? It is filtering the water, maintenance regularly done. I've tipped it to eliminate air. I've had it approx 8 years. Been a great filter. The sound only happens when the filter is running. Any ideas? I feel bad for my fish.
Thanks
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