Massive clumps , prior to the formation of any visible protostars , are the best candidates to search for the elusive massive starless cores .
In this work we investigate the dust and gas properties of massive clumps selected to be 70 \mu m quiet , therefore good starless candidates .
Our sample of 18 clumps has masses 300 \lesssim \mathrm { M } \lesssim 3000 M _ { \odot } , radius 0.54 \leq \mathrm { R } \leq 1.00 pc , surface densities \Sigma \geq 0.05 g cm ^ { -2 } and luminosity/mass ratio L/M \leq 0.3 .
We show that half of these 70 \mu m quiet clumps embed faint 24 \mu m sources .
Comparison with GLIMPSE counterparts shows that 5 clumps embed young stars of intermediate stellar mass up to \simeq 5.5 M _ { \odot } .
We study the clump dynamics with observations of N _ { 2 } H ^ { + } ( 1 - 0 ) , HNC ( 1 - 0 ) and HCO ^ { + } ( 1 - 0 ) made with the IRAM 30m telescope .
Seven clumps have blue-shifted spectra compatible with infall signatures , for which we estimate a mass accretion rate 0.04 \lesssim \mathrm { \dot { M } } \lesssim 2.0 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } , comparable with values found in high-mass protostellar regions , and free-fall time of the order of t _ { ff } \simeq 3 \times 10 ^ { 5 } yr .
The only appreciable difference we find between objects with and without embedded 24 \mu m sources is that the infall rate appears to increase from 24 \mu m dark to 24 \mu m bright objects .
We conclude that all 70 \mu m quiet objects have similar properties on clump scales , independently of the presence of an embedded protostar .
Based on our data we speculate that the majority , if not all of these clumps may already embed faint , low-mass protostellar cores .
If these clumps are to form massive stars , this must occur after the formation of these lower mass stars .