Sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond have emerged as promising candidates to characterize ultrathin and 2D materials. An outstanding challenge to this goal is isolating the contribution of 2D materials from undesired contributions arising from surface contamination and changes to the diamond surface induced by the sample or transfer process. Here we report on such a scenario, in which the abrasive deposition of trace amounts of materials onto a diamond gives rise to a previously unreported source of magnetic noise. By deliberately scratching the diamond surface with macroscopic blocks of various metals (Fe, Cu, Cr, Au), we are able to form ultrathin structures (i.e., with thicknesses down to <1nm), and find that these structures give rise to a broadband source of noise. Explanation for these effects are discussed, including spin and charge noise native to the sample and/or induced by sample-surface interactions, and indirect effects, where the deposited material affects the charge stability and magnetic environment of the sensing layer. This work illustrates the high sensitivity of NV noise spectroscopy to ultrathin materials down to subnanometer regimes—a key step toward the study of 2D electronic systems—and highlights the need to passivate the diamond surface for future sensing applications in ultrathin and 2D materials.