We're in the middle of a cold spell in the North East. I have an oil furnace. It is heating the water for the heating system okay. However, it is making strange noise.

When the heating system is running, normally it makes a nice solid roar. However, now it goes back and forth between relative quiteness and the roar. (Almost as if its taking a breath.) The flame is solid and strong the entire time. And, as I said, there is hot water in the radiators.

Does anyone have any ideas as to what it might be?

I have a service scheduled for next week. Should I wait for the service, or should I get someone sooner? And do you think its okay to run the furnace?

Any advice will be much appreciated...
Conor.