Question: On a recent trip to Eretz Yisrael, our tour guide warned us not to place our fingers into the stones of the kosel. Are we not allowed to place a note inside or touch the stones?
Answer: According to the Mishna (Kelim 1:8) it is forbidden mideoraisa to go into the place of the beis hamikdash while tamei (See Zevachim 32b).
There are various midrashim (Shemos Rabba 2:2; Bamidbar Rabba 11:2; Midrash Rabba Shir Hashirim 2:9:4, et al.) that talk of the uniqueness of the Western Wall of the beis hamikdash. Based on this, the Radbaz (2:648; 691) believed that the kosel is a remnant of the beis hamikdash itself. Thus, the Chayei Adam (Mishpatei Eretz 11:8) writes that one can walk close to the kosel though one shouldn’t touch it (See Yabia Omer 5:YD:27).
However, the Avnei Nezer (YD 450), R’ Yechiel Michel Tukachinsky (Ir Hakodesh Vehamikdash 4:2) and R’ Ovadia Yosef (ibid) write that the kosel is not a wall of the beis hamikdash but a retaining wall of the har habayis, Temple Mount (See Tzitz Eliezer 10:1). R’ Ovadia Yosef concludes that those who are machmir not to touch the kosel are mistaken. Similarly, R’ Moshe Feinstein (Igros Moshe OC 2:113) writes that R’ Yitzchak Zev Soloveitchik (the Griz) wouldn’t visit the kosel though he doesn’t understand why one would avoid it when it has been the practice to go for so many years.R’ Shlomo Zalman Auerbach (Minchas Shlomo 3:160:3) writes that although the wall surrounds the har habayis, one may place one’s hands into the wall. Thus, there is no problem with placing a note in the wall.