It difficult to say what the sound was without being in the car. All I can say is this- change gear when the engine sound feels distinctly sharp. This usually happens around 2k-2.5k rpm. But that it- engine sound, not car sound. In the Swift, I always had the problem wherein the car made a hollow sound when going over a paver block road. No matter what speed, as long as you were above 3rd gear, it always made that sound. That's not an indicator for dropping to a lower gear though.
If you are new to driving, use the speed to gauge gear- so if you are at 20 in 3rd gear and the car sounds strained, drop to a lower gear.
But from first hand experience, that not very critical in the Swift. I have been at 30 in 4th and 40~45 in 5th cruising on a level road several times without any complaint from the engine. That engine has more than enough torque to handle load provided you listen to what the car is telling you
Ameyam