How freqent are the "tacka" sounds? Like >1/sec? If this is the case its probably the ABS pump making the noise. You can get a automotive stethescope or just use a screw driver held up to your ear to CAREFULLY listen to various components while you hear the noise to verify.

The ABS/ASR pump on my car would make a "tacka tacka" clicking noise on start up and following any ASR/ABS engagement. I found that the pump was bad and replaced it for ~100 (used eBay purchase)

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE