Intermodulation interference is produced by:

Receiver intermodulation interference happens when two or more strong signals enter the antenna and reach the receiver's front-end components, such as the RF amplifier or first mixer. If these components are pushed into nonlinear operation by the signal strength, they act like an unintentional mixer. This accidental mixing generates sum and difference frequencies (intermodulation products). If one of these new products falls on the frequency you are trying to listen to, it causes interference that cannot be filtered out by the subsequent stages. It does not typically occur in the IF amplifiers because the initial front-end filtering and the first mixer usually restrict the bandwidth, meaning the multiple original off-channel signals never make it to the IF stages together to interact.