Joined: Jan 2010
Posts: 9,809
Likes: 1,783
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
Maybe a mud dauber nest in there?
A drill will show you what's in there, or maybe just different diameter rods, might get one to break through.
The WD40 would prevent the mud from being softened with water.