A/C SYSTEM

You may be hearing the air conditioning compressor clutch cycling on and off which is normal. Could you not have been aware of the sound before or is it a noise which has just occurred? Safest to get it checked out, ask a mechanic to have a listen and get their opinion.

I'm with Patrick, although the rattling sound concerns me at speed with a possible compressor clutch / coil / pulley issue. A wobbling pulley will make a rattling noise. Definitely do get it checked ASAP.

I'm not so sure that the compressor is the culprit. It would cause noise on the passenger side not the drivers side. Because of the way the engine is mounted. I'm wondering if you might have a broken blend door in the heater box. If you are running the A/C with inlet air on the speed of the air along with the Blower motor are causing enough air flow in the box to rattle the broken door in the case.