have you matched the dimmer to the load type? 23W lamp sounds like a CFL, most of which are incompatible with dimmers. 75W sounds like an incandescent lamp, should be dimmable on most typores of dimmer, but some older dimmers needed a neutral connection to avoid a noise as you describe. And some CFLs make a low-level hum / buzz even without a dimmer in circuit, possible due to harmonics on the line.

Hi Liz, Alec's right about the lamps - if you are using a CFL-i make sure its a dimmable one - even then it may make some noise. A lot of older or cheaper dimmers, based on triac switching technology are prone to making noise. Have a look at the HowStuffWorks website on how dimmer switches work for a detailed explanation. Basically if its the dimmer thats making the noise - you should try replacing it with a modern trailing edge dimmer. If the lamp is making the noise try changing the lamp for a new generation mains voltage halogen bulb.

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Best regards

Jack