What event do you wish to detect and trigger your hooter?
Is it local to your network or somewhere else?
Is it connected to the internet or mobile phone network?
How is the event initially detected? What hardware is involved?
What hardware are you running Node-Red on?
"Can you do it by reading value from webserver device?" Yes you can.
Edit - You asked how to do this. Sorry, I have no idea. I never use HTTP like this since MQTT suits my requirements and seems simpler.
"How do you use MQTT?"
There has to be an "MQTT Broker" running 24/7 somewhere.
It could be on a small local device such as a Raspberry Pi. There are free/cheap brokers in the cloud which might be suitable if the event you want to detect is not within your network.
Let's assume your alert is a nuclear reactor meltdown.
The device which monitors the reactor connects to the broker and publishes a message using a topic "reactor/event" and a payload "meltdown".
In Node-Red you have an "MQTT In" node which connects to the broker and listens for messages on the topic "reactor/event". When a message "meltdown" arrives you turn on the relay for the hooter.
The broker has to be accessible from both devices, the one which publishes the message and the one which listens for it. Hence the questions above.
