This problem has existed with all the nokia maemo devices since the 770. To fix it you need to install a Ground Loop Isolator in the headphone out cable.

Most Isolators are based on Phono Connectors, but the PAC SNI1/3.5 comes with 3.5mm jack connections.

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I haven't tried one with my N900 yet, but I've got a long journey coming up so will be buying one soon.