I'm going to say that it's a bearing problem. When bearings go bad it's not always a constant sound. The sound from bad bearings depends on how the load is applied to them. For example, a bad bearing unit could make noise only when turning in one direction, or only when braking; when a particularly heavy load is on them. Ok, I'm not saying that it's definitely bad bearings, but this is my experience from bad bearings in my Volvo, 300D and Landcruiser. Oh, and when a CV joint went bad in my Volvo 850, it made a clicking sound.

Alex

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1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside