1999 Ford Taurus Engine Sound

1999 Ford Taurus mileage: 108,000, V6. Purchased this car from a Dealer a weak back. He told that he did some transmission work to this car recently. But I noticed from few days ater the purchase that that it sounded as if it was struggling slightly when I was going between 30 mph to 45 mph. Its clear that, car is struggling and I can notice some engine sound. But above 50 mph, it acts normal, but when under 50, I can notice engine sound which is clearly visible to all inside the car. Transmission fluid full and red, no smell of burning.

When I start the car, I cant notice any sound. Its normal. But between 30 to 45 mph I can clearly notice the Engine sound.

Can you please assit?