Timing chain hydraulic tensioner do this very often. The hydraulic cylinder separates inside the housing. DO NOT drive the car like this, it could cause a serious damage. Beside this, the 1,8T engine is notorious about building up sludge, causing all type of internal engine problems, so I wouldn't be surprised if there are additional problems existing. Find a good VW specialty shop in your area and let them diagnose it.
While a timing belt is driven by the crankshaft pulley, there is, in fact, one hydraulic camshaft chain tensioner or cct for the 1.8t 4 cylinder engine camshaft chain and two total or one for each camshaft chain for the 2.8 aha or atq v6 30 valve engine. These often cause noise either due to worn or cracked/missing plastic cam chain guide pads, or due to dirt or sludge in the oil causing noise and sticking in the hydraulic cylinders of the cct's, creating tapping noises especially at idle and low rpm.
It's hard to guess at the source of the noise without being able to hear it first hand and see if it is at engine speed or half engine speed, or if it internal to the engine or external to the engine. Check the engine oil level first. Definitely get it looked at even if it's just to see if it's safe to drive, if it internal to the engine (something starving for oil) it will get worse and more expensive to repair if left neglected. That engine it is really important to change the engine oil and filter frequently and use synthetic oil. I have seen these engines fail prematurely due to infrequent oil changes.