potential new owner..tapping noise

I have not owned a discovery before and am looking at a 2002 SE with 71,000 miles. I have read about possible problems with head gaskets, transfer cases and fluids and checked the car thoroughly both when stationary and driving.
I did hear a small knocking noise once it warmed up which sounded like it was coming from behind the steering wheel. When I lifted the hood it was possible to hear this from that side of the engine. The rest of the vehicle looks to be in very good condition. Can anyone tell me what this noise could be.

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