What could cause the specified pulsating/shutter issue on my vehicle.

My vehicle is a 2005 Pontiac G6 with 180k miles. I just recently purchased it as a commuter car. I drive 200 miles a week in this car to and from work. About once a week a will get this odd pulsating/shutter(du du du du) noise/feeling when I am accelerating in 3rd or 4th gear. It has never happened in any other gear and only happens in 3rd when it is about to switch to 4th. The motor runs well, so I am thinking that this is likely a transmission issue. However that Catalytic converter is worn out, so I am thinking that this could POSSIBLY explain this.

To be clear, whenever I am cruising about 55MPH in 4th gear at 1500RPM and I decide to punch it, I will usually get that noise/feeling. If I let off of the accelerator for a few seconds and then lightly accelerate, it is fine. If I am cruising at 75MPH and accelerate to 3000-4000RPM, it will generate that noise/feeling(as soon as I let off for a few seconds it goes away).

I am going to be changing the tranny fluid out on it and putting some lucas tranmission fix in there with it this weekend. I have not received any Check engine lights that would indicate a transmission issue.

Have any ideas on what in the transmission can cause this? OR Any ideas on whether or not the Catalytic converter can cause this?

I am leaning towards an Overdrive issue or chunk of metal sitting in the bottom of the pan that gets sucked into the gears from the pressure increase or the 4th gear clutch might be worn.