A quick google gives you this link.
Basically the subframe moves fractions of a mm, mainly at low speed during parking maneovours. The stretch bolts used "stretch" over time.. often you'll find yourself able to do them up with a socket from underneath..then in a month or so they'll become "loose" again.
When the subframe moves it creaks or grinds shall I say on another part of the cars chassis which causes the cracking noise.
Especially common in lowered Mk5s but can happen to any and all of them regardless of type, TDI, GTI, R32 etc.
New bolts and a small shim to space the subframe away from the chassis stops the contact and thus the noise. Originally made by ECS but now available from VW directly.
2x N90823501
2x Shim 7M3499349A
Available from VW, TPS, or online vendors such as Darkside Developments. Last time I checked Darkside were cheaper than VW.
Can be fitted on your driveway if you have axle stands (as you need to get under the center of the car) or if you can get it on a ramp it takes 10 minutes.