You mention barriers - might it be assumed that there are roadworks in the area ?

If that is the case and there are also temporary lights or traffic lights in operation, the contractors may be using a silenced generator to provide power.

Unlike the old days of noisy monster generators, the modern equivalents are very much quieter.

They are not absolutely silent however and even when you were standing beside it you would hear a very low frequency hum although it may not be clear how far the reduced hum would actually transmit distance wise.