If a battery is under 80% of full charge (my guesstimate) some gassing noise is normal. Violent bubbling- no.
The real test is to measure the charging voltage. On float, which means the batteries are nearly fully charged, the terminal voltage should be 13.2-13.4V. When the batteries are being brought up from a lower state of charge, a charger that features bulk and absorption phases can put out higher voltages in the range up to about 14.2V. An equalization phase is probably about 14.4-14.6V and that voltage is only applied for a couple hours. These voltages only apply to flooded wet cells.
The bottom line- you need a decent ($30) digital voltmeter to evaluate your battery charging.