Is the following that you want to achieve?
1. If (and only if) the data byte 0x0D comes from the InputBox of the Serial Monitor into the UNO, the UNO will send the message "SENT" to the Serial Monitor which will appear on the OutputBox of the Serial Monitor. To send 0x0D (ASCII code for Carriage Return) from the InputBox, you activate the option Carriage return in the formatting box (bottom of the InputBox) and click on the Send button.
2. If nothing is sent from the InputBox of the Serial Monitor, the UNO will keep sending 0x05 to the Serial Monitor. Because, 0x05 is an ASCII code for a non-printable character; so, change it to 0x35 and insert a delay of 1000 ms. You will be able to see that character 5 is being printed at 1-sec interval on the OutputBox of the Serial Monitor. You may remove the line (s) Serial.flush().
The slightly modified codes:
if (Serial.read() == 0x0D)