Silicon is a known trace contaminant in diamond grown by chemical vapor deposition (CVD) methods. Deliberately Si-doped diamond is currently attracting great interest because of the attractive optical properties of the negatively charged silicon-vacancy (SiV-) defect. This work reports in-depth studies of microwave activated H2 plasmas containing trace (10-100 ppm) amounts of SiH4, with and without a few % of CH4, operating at pressures and powers relevant for contemporary diamond CVD, using a combination of experiment (spatially resolved optical emission (OE) imaging) and two-dimensional plasma chemical modelling. Key features identified from analysis and modelling of the OE from electronically excited H, H2, Si, and SiH species in the dilute Si/H plasmas include (i) fast H-shifting reactions ensure that Si atoms are the most abundant silicon-containing species throughout the entire reactor volume, (ii) the low ionization potentials of all SiHx (x ≤ 4) species and efficient ion conversion reactions ensure that even trace SiH4 additions cause a change in the dominant ions in the plasma volume (from H3+ to SiHx+), with consequences for electron-ion recombination rates and ambipolar diffusion coefficients and (iii) the total silicon content in the reactor volume can be substantially perturbed by silicon deposition and H atom etching reactions at the reactor walls. The effects of adding trace amounts of SiH4 to a pre-existing C/H plasma are shown to be much less dramatic, but include (i) a Si substrate or fused silica components within the reactor is a ready (unintended) source of gas phase Si-containing species, (ii) OE from electronically excited Si atoms should provide a reliable measure of the Si content in the hot plasma region and (iii) Si atoms and/or SiC2 species are the most abundant gas phase Si-containing species just above the growing diamond surface and thus the most likely carriers of the silicon incorporated into CVD diamond.