Don’t use too much torch gas when welding aluminum on AC. Aluminum takes a lot of amperage to weld. Even though the melting temperature of aluminum is less than half that for steel, it takes about twice as much amperage to weld.
Why? Because aluminum conducts heat away from the weld puddle faster than you can put it in. this brings me to an important point.
Do not use more argon than necessary on your torch gas. If you do, it will be like blowing cool air on something you are trying to heat up with a torch. All that argon blowing on the part makes for a loud erratic arc because the arc force is so great. Have you ever lit up on a thick aluminum casting and listened to how loud the arc is? I bet your torch gas was up around 20 like the books recommend. That’s too much for aluminum (unless you are using an argon helium mix).
And another tip is use the old school type of collet body(not gas lens) and one size smaller cup than you would use for steel that still provides good shielding. A smaller old school (not gas lens) TIG cup confines the shielding gas envelope to the puddle so that arc energy is not wasted in the form of frosty cleaning action outside the weld. A lot of Old timers use the small cups, they just don’t know why.
Pay attention next time you weld aluminum and use a small cup and then turn the shielding gas flow down to around 12-15 cfh with a #6 cup and see if things don’t quiet down a bit.