With the cooker cold switch it off at the cooker switch, Unscrew the fan guard from inside the oven and check the fan turns without touching the element , if the fan catches it you can adjust the element slightly by slackening the screws and re tightening them . take into consideration the element expands when it is hot , just make sure there is enough room all the way round the fan , re assemble it , switch it back on and listen again , hope that cures it as i had a similar problem .