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Does the knocking get louder/speed up when you step on the throttle and make the RPM go up?
Most likely you have a bad rod bearing if it increases with RPM's. That's not good and it's usually not worth fixing the engine because when that knocking happens the rod is banging against the crank journal and the crank needs to be taken out and turned on a lathe. It's cheaper to get another engine from Mazda or a used one from a wrecker and install it. You could take tow it to Mazda and see if they'll help at all they may offer assistance but I'd not mention anything about oil not being in it.