The phrase "5 volt rectified AC voltage" is gibberish.
If you have rectified it, it isn't an AC voltage. The electron flow is no longer alternating in direction.
You will expect to see a DC voltage with a voltage ripple. Which is exactly what you are getting.
Depending on the type of rectifier, you will see 65 Hz or 130 Hz ripple. I can't tell from your
sampling rate, exactly which you are getting.
Your DC voltage has a peak of 5 volts and a minimum of about 4.5 volts. That is exactly what you
are getting.